Disappointment and heartache is all part of growing up. Sorry, Cyndaquil.
Oh, I share your pain, Typhlosion. I, too, would have loved for more started to get mega evolutions.
IMHO Mega Evolutions just ruined the games.This is just for the better.
Z moves aren’t replacing mega evolutions, they’re replacing type gems. We had to go through a generation with all but the normal gem removed already.
Basically they work the same as type gems, with a few exceptions. First, you get only one use per battle. No recycle, no having it on multiple pokemon. Second, you can use it at will instead of only on the first attack of that type, so if you want to use a move normally instead of boosted you can. Third, the held item is not consumed, meaning the acrobatics/flying gem synergy is gone (doubly gone, see #4), and you can’t use a gem then have room to steal an opponent’s held item afterwards. Fourth, the attacks themselves can differ from the base attack. The power is not exactly double the base move, but rather a nonlinear scaling. Accuracy and secondary effects differ, and it may or may not inherit the damage category depending on the move. Additionally, unlike gems which only work for offensive moves, there can be Z status moves like eevee’s extreme evoboost. Finally, move compatibility can be restricted, such as for Snorlax who can only boost giga impact if using the snorlax-specific Z crystal (but you can use the normal-type crystal instead and boost anything else if you prefer)