Hmm, I see where you come from. An interesting point of view, and while I somewhat agree with what you said about the concept mutating into "making a pokemon broken for the heck of it", I still think megas aren't exactly losing the spirit of a mega, is just that Gamefreak's decisions on the pokemon who receive a mega that sort of bug me.
For example, I personally dislike the fact that they gave metagross, garchomp, rayquaza, mewtwo, etc. a mega (mewtwo even has two...). I like the megas themselves in terms of design, but what I don't like is that they took some of the already most powerful pokemon and made them so overpowered that you can hardly counter them in competitive. Whenever I decide to play just for fun with a team that has no specific counters for these threats, I always find a team that has a mega powerhouse (lucky me -.-), and all I can do is just watch the opponent sweeping my team with one single mega. Even given how terribly bad I am at competitive, I can still enjoy losing to another person when they use strategies that make me say "woah" or have incredibly well-thought teams, but when they 6-0KO me because they had a mega-evolved powerhouse, well, I just find that kind of match boring. And when it is me who gets to use one of these goddamn powerhouses, I find it boring to 6-0 KO the opponent's team. A Vivillon sweep? I take it. A mega-rayquayquay sweep? Meh. That's my opinion on the competitive side at least.
However, if I love the concept of mega evolution is for one reason and one reason only: it seems to be the only way now for pokemon that were seemingly abandoned by gamefreak to receive some love, given that trans-generational evolutions (like gliscor, weavile, froslass, dusknoir, etc.) aren't happening anymore for some reason. The best example, Audino. No one likes Audino (I love it but that's besides the point). The thing is, the mega is not overpowered by any mean, and it won't make people start loving it, but at least the fact that a mega audino exists shows that Gamefreak cares about that pokemon. That's all I need, to know that it isn't abandoned into oblivion. If things like Leavanny, Dunsparce, Flygon, Spinda, etc. received megas that would be great just for the fact that it shows that they're not forgotten. That's the reason why I love things like Mega Bedrill and Mega Pidgeot. No one used them, and Bedrill was specially forgotten, but now? It gets some love in the form of what I think it is one of the best mega evolutions design-wise. I prefer regular evolutions a hundred times, but if mega evolution is the way to go now, and it's the only way some of my favourites receive some love, then I'll take it.