Well, to me, mega-evolution (and its cousin primal reversion) brought both good and bad things to the franchise and the meta. Since I'm no meta expert or good competitive player to begin with, I won't be talking about the meta much, only gonna say that I do think megas over-centralize team-building: you're better off with a mega than without a mega, and so unless you're playing a tier that doesn't allow megas, you always have more reasons to use one than not to use one. I don't have more thoughts on them in that regard.
To the franchise as a whole tho, I think megas have both good and bad consequences for the house (Nintento-GF) and the fans. First off, they offer a new window of potential, as they bring the possibility to open up evolutionary families that were "closed" or "complete" by having 3 stages, to a whole new level that includes a final, 4th temporary form that is stronger than any of the previous 3. I kinda like this because it pushes the boundary of "pokemon can evolve up to 2 times", that we all have learned over time, far and beyond expectations and allows for flows of creativity that wouldn't have been possible not even for the wildest fakemon creators (arguably, I've seen guys who have come up with pretty awesome fake stuff). Since it is temporary, it makes sense that these 4 stage evos don't stray too much of the original pokemon, and only add a few characteristics that can be considered as improvements of the original (I'll get to that in the blink of a sableye. Get it? cause it has no sableyelids? ...).
The other side of this coin however, is that mega-evolutions have the ironic consequence of being able to lock evolutionary lines as easily as they can open up others. Take standalones like Absol, Sableye and Mawile, for example. They have megas, cool, but that implies they won't evolve further naturally. You'll never see an evo (or prevo) to absol, sableye or mawile, they'll never turn into evolutionary "families" in the sense they've been locked by their own megas. Sometimes this is good when a design can hardly be expanded more upon in order to create an actual evolution (to me, this would be Absol's case), so you give it a mega, that only adds extra details or slightly different shapes, and you have something completely different except it is not, and you also made an evo to a mon that no one believed could ever evolve, everybody wins. Other times, it might be seen as "bad" or wasting potential, like in the cases of Sableye and Mawile, mons with a very low BST and that have a whole world of possibilities and creativity inside them: Sableye can still be made bigger, more menacing, more evil, more awesome, and Mawile can be made bigger, and have more jaws. Now their megas exist, this world of possibilities is water under the bridge, is mere "what if", a "fakemon matter". Sableye is locked into two possible looks: he's either a tiny purple man with blue jewel eyes, or a tiny purple man with red jewel eyes and a giant rupee shield. Mawile however takes a different approach, and in all honestly, it's mega form could easily have been made into an actual, legit evolution, allowing it not only to have greater stats than 480, but also been less broken without being actually any less powerful. By taking the clever approach of adding more jaw-horns and a red body-color, mega-mawile kinda borrows a bit of that world of possibilities that it itself locks by the mere fact of existing.
My wishes for this specific point is that GF took the time to consider if mega or regular evo is more worth it before starting the "mega-boom" period. Take Marowak, Lanturn and Dunsparce, for example; mega or evo? is not just about jumping straight into the mega-wagon and go "megas for everybody!", but it isn't about being anti-mega either. Take your time, analyse the mon's design, capabilities, what can it do? what can it not do? what does it need? 100 extra points? a secondary type? a new ability? more moves? can its design be explored further, or can it only be added tiny details to? I think that before trans-generational evolutions competely disappear to megas, it should be necessary to consider what benefits the mon more, and act accordingly. The franchise should be smart enough to keep both.
Leaving that point, a thing I thank mega evolutions for is bringing more love and usage to mons that everybody forgot about, or loved but is a love that hurts, because they sucked in game or competitively. Again, let's pick the 3 previous examples for this: Absol is the mon everybody loves, but is too fragile and slow to be any good in common play. Mawile is very interesting graphically, but competitively it is weak to too many things for its low BST and stats, and isn't really worth much to play with, there were better steel types. Sableye has always had a niche as a prankster annoyer, but its fragility made it fear priority and strong attackers that can't be burned or affected by its prankster antics. Mega evolution changed that. It opened a new niche for these pokemon. Mega absol is fast and offensive and can't be burned, mega mawile is TOO good with 105 attack plus huge power and its new steel/fairy type, and mega sableye is a wall that can't be burned, taunted or poisoned (even though it has low HP to withstand hits). Overall, it can be said mega evolution brought improvement to all these mons, and many others: who ever used beedrill? who loves pidgeot but the regular sucks? glalie? loppuny? audino? they all get more usage now (on average), thanks to being given megas, specially those that sucked even after being the result of 2 successive evolution processes, like beedrill or pidgeot.
However, this also brings a "bad side of the coin" like the previous point did: bad adiministration of mega-evolutions. Giving something that is already powerful or has a rather exploitable niche just more raw power and a better ability to appeal to fans or just to generate hype. Even more. Giving something TWO megas instead of one, just to appeal to fans more. Sometimes you don't need to give more love to something fans already love. Give it the right amount of love you give the rest pokemon, but don't make it better for the sake of even. An example: metagross is a pseudo, that means it is strong. But the nerfing of steel types that are now neutral to dark and ghot, strong weaknesses on fire and ground that only grow greater every new gen with the addition of stronger STAB EQ and Fire blast users.... regular Metagross has reasons to get a mega. It might be too good, just like mega-mawile, but is fair, the guy needed it. Okay. Now take a look at garchomp. Its nerfing was the apparition of fairy types, and the lack of perpetual sandstorm via sandstream users. But here's the thing: Garchomp is good. Too good. Even with changes to its environment, it still manages to be OU, and other things can deal with fairies, and it doesn't need sandstorm all the time. Garchomp is still good. Now why do you give it a mega, if not to appeal to fans? there's no other reason. Even more, as it is slower than the regular, and is limited to a single ability, mega garchomp has a smaller niche than regular garchomp, ironically enough. That's a mega that was uncalled for, in my opinion. Then you have mewtwo, who being one of the strongest legendaries gets not one but two (both strong as fucking hell) and to top it off, charizard is indirectly "touched by angel wings" by also being given two megas instead of one, unlike the rest of mundane starters, because everyone knows that charizard is much more of a fan favorite than other starters, it "clearly" deserves 2 boosts. Welp, no. Blaziken and Blastoise are as much fan favorites depending on who you ask, and yet they manage well with 1 mega. If anything, you know what deserves 2 megas? Meganium. Don't even get me started in mega rayquay quay that can hold items, and primal groudon that can negate its main weakness completely...
To sum up my thoughts, I think megas should be for everyone, as long as is given the mon gathers the right requirements of not being insanely powerful already and needing a boost that is better suministrated with a mega than with a regular evo. Lots of mons enter that category: Cloyster, Flygon, Weavile, all starters (that greninja tho), rattata tropes, pidgey tropes, water type fillers, liepard, etc. etc.