You use the word "fanservice" like it's exclusively a bad thing. In some cases, it might be, but in Pokémon's case, it's face-meltingly awesome and A-OK with me.
Like Green Thunder and Whitman Greninja'd, Mawile's a perfect example of a relatively weak fan-favorite that benefits greatly from Mega Evolution.
And Charizard? Top tier? Really?
Charizard is a Top-Tier Threat in NU due to it's Ability to 2HKO virtually everything with Solar Powered Boosted Attacks. And as of it's reference as a "shitmon", that's funny because Charizard receives Usage in just about all the Upper Tiers, above some that are considered better than it. This shows that it has at least some Viability as a Sun Sweeper; an unprepared OU Team can find itself torn apart by a Charizard with Rapid Spin Support.
Good job on finding literally one Pokemon that was considered Weak before Mega Evolution, lol, but 1 out of the several that received Mega Evolutions were based off of Popularity and "what the older fans want", instead of giving Pokemon that deserve it more such as Corsola or Butterfree more Viability :(
Both of these Pokemon are fairly popular, why didn't they along with Mawile receive a Mega Evo?
Although I understand where your coming from with the "fanservice is good" argument, I'm going to have to disagree in this case that fanservice was the best route to go because upon that, we arrive at my point yet again: Mega Evolution isn't
necessary, it is instead a Fanservice that results in the destruction of an already defined Gameplay with the recent Type Chart Update. In fact, Mega Evolution contradicts itself in that by powering up Fan favorite Pokemon, they actually overshadow and distort the image of the original, which can cause the Older fanbase to actually dislike it rather than be attracted to it.
But none of that is relevant, as the point is, "Mega Evolution is completely unnecessary Game Mechanic" as we have gone through 5 Generations in 15 Years with a strong, growing fanbase. Its presence can actually cause a decrease in fan base as well. And just because you like it, because its a "A-okay" to you or whatever, that doesn't speak for everyone.
I feel That I'm sending the message that I dislike Mega Evolution as well. I do not particularly dislike it, I just don't feel its necessary- Aka I'm neutral on the subject. However, what I DO dislike is what Pokemon are receiving it, :the
majority of who were just fine without them, while other Pokemon who need it are just flat out ignored.