This has probably been asked a billion times before, but was there really any reason for Mega Evolution to be removed from the series?
Actually, you're asking the wrong question.
Was there any reason for Mega Evolution *to stay* in the series?
From what we know, mega evolution wasn't even going to be included early in SM, it was only added later as post-game only thing.
Considering all "regional gimmicks" are, well, strictly tied to a region, it was likely already the plan to not go forward carrying them over, just due to the popularity they opted to keep them in. Which in hindsight was a terrible idea, because it created the expectation that mechanics would carry over, and when they and Z-moves didn't show up in gen 8 games, people felt betrayed by a promise that was never made in first place.
(Ironically, same as movesets "not carrying over", despite movesets haven't actually been carried over for 4 generations now but artifically been carried over by transfers)
Without it, there is absolutely no reason to have shitmons like Sableye and Mawile in the games, which is even further emphasized by the fact Butterfree got a GMax form in SW/SH and Beedrill got banished to the Distortion World (AND STILL HASN'T RETURNED).
This is irrelevant, because you're just replacing a mon with another mon. Besides, it didn't *really* help that many mons anyway, because due to the opportunity cost of using a megastone, ultimately only the top X megas saw play, everything else still got abandoned. Shotout to CHALK definitely making any mega not called Charizard, Metagross or Kangaskhan see play.
The meta wasn't as f_cked as people say it was, if you saw someone running, say, a Beedrill and a Charizard, there is an approximately guaranteed chance the Beedrill is going to Mega instead of the Charizard due to the fact that normal Beedrill is unfathomably bad compared to normal Charizard, hence you could plan around it.
Same as above, people consider the mega as it's "i'm playing the shit pokemon", but no, you're not playing "charizard", you're not playing "sableye". You're playing Mega Sableye. They're not different from alternate forms at all, expecially as except extremely fringe cases to swap in a immunity before changing type or keeping the baseline ability for the free turn you get by swapping in, you'd always mega evolve istantly as soon as the pokemon hit the field, so you shouldn't illude yourself that "Beedrill saw play", because it didn't, Mega Beedrill saw play, Beedrill was still as useless as it always was.
I just really want Mega Beedrill back.
This i can respect, but hey, you can always hope to see them again whenever gen 6 remakes happen. In like 50 years.