I don't like Mega Evolutions at all. This latest batch has made me question the concept and of its permanent effects on the future of the franchise.
And why is that?~
I second what @Team Gaara; said.
Mega evos are awesome yo. I'm not a fan of them releasing new evolutions for old Pokemon, as that risks ruining Pokemon for people. For example, I would die if there was a Sableye evo, for various reasons, and other people are also pissed off when Pokemon they like get evolutions they don't like. With megas, that threat isn't there nearly as much, as if you don't like a Mega, it's perfectly viable to simply not use it in favor of another Mega. Also, it's temporary, unlike regular evolution.
Megas also breath new life into old Pokemon, which is nice. A lot of people who like Mawile now became interested in it because of its Mega, for example.
I'm quite confused about why some people dislike the concept. Less experianced players often accuse them of being "OP", which is an incorrect conclusion drawn from shitty assumptions. Sure, some megas are overpowered, but most aren't.
Why do these new three in particular make you question the concept? The only reason I can think of is that you dislike their designs, which shouldn't make you upset with the concept of mega evolution itself, just the designers.
For the record, I personally think the designers are doing a great job with Mega Pokemon. For the most part.