You're right that mega evolutions were great in that it gave certain Pokemon a second chance to be useful however even that took a wrong turn when they mainly focused all mega evolutions to to all the popular Pokemon who were already great to begin with. On top of that, it did continue forward which I feel would have been another opportunity to give those not-so-great pokemon that chance.

In a vacuum Megas were fun, but I think they were a net negative to the games as a whole.

In the single-player game, it's basically an instant-win button. I don't think pokemon needs to be difficult- I find most rom hacks and nuzlocke runs tedious and annoying. But having one button that just wins felt bad. The whole concept of temporary transformation felt like something that didn't belong in pokemon, probably because its absence was something that separated Pokemon from competitors like Digimon and Yu-Gi-Oh.

Before Megas, new mechanics were usually things that made sense. Things that fit neatly into the world, may have been in the anime early, and were pretty logical conclusions that were only not in earlier games due to technical limitations. Splitting Special into SpAtk and SpDef, splitting moves within types by Phys/Spec, adding Steel type, Held Items, abilities, double and triple battles, breeding. To me, Megas felt drastically different, as do Z moves, Dynamaxing, and Terastillizing.