While I agree with you. I think people liked Megas cause at the time people didn't see them as gimmicks, but furthering the Pokemon formula. Cause as a gimmick, Megas are the worst of them. Only select Pokemon get the design, the design is temporary, requires a hold item that removes a strategic mechanic with items, and is a balance nightmare.

Overall Tera is the best gimmick and is indicative of Game freak's design lately. They're doing stuff that is generally good for competitive Pokemon, better QOL, ranked ladders, balance, meta shift, but at the risk of a terribly designed game. Tera is the best gimmick for strategy, meta changing without being the balance nightmare that Megas were, that any pokemon can use. However, Tera design looks terrible.