I haven't spoiled myself on the SUMO story yet so maybe this is getting off topic, but is it really that much of a recon?
Mega evolution had some sinister overtones in XY and ORAS, since the stones were formed as by-products from either an ancient superweapon or a meteor. And it's all implied to run off of "infinite energy" which Sea Mauville pretty much stressed was completely unethical.
So Mega-Evolution being powered by hopes and dreams seems like was always just us assuming typical anime tropes. Unless hopes and dreams means the lives and souls of pokemon given to make the mega stones. It always had this grim undertone, they were just really subtle about it in Gen 6.
Weather you like this explanation or not is your own opinion, but just pointing out mega evolution was always a bit messed up.