I agree that it sucks, but at the same time, if some of those Mega forms became real evolutions, there would be an insane power creep like no other.It's what I expected but I still hate it. Dynamaxing on its own is rather inoffensive as a mechanic even if it is mind-numbingly boring and dumb (in normal battles; the raids look neat enough). But to lock what are ostensibly evolutions behind it is pretty much encouraging me to not use Pokemon that have Gigantimax forms considering I can't take them forward. Not that this seems to matter anymore, but at least with normal-ass evolutions there's a chance I can use them in future regions.
We can only hope that Pokemon who desperately needed those Mega Evolutions to be competitive (like Mawile with its very poor base stats and useless abilities) can get proper permanent evolutions down the road to make them better.
For what it's worth, besides the expected remakes, I can also see Gigantamax carrying into Gen 9 if Switch is still the target console. I'm sure they want those new assets to last as long as possible.