i think the reason i like mega evolution so much more than the other game mechanics that have been introduced since is that it essentially represents the main theme of the entire franchise-- the bond between people and pokemon. mega evolution is explicitly stated to only be possible if a pokemon and trainer are deeply connected, and thus the power and appearance gained through it are a physical portrayl of that connection. it's more than a game mechanic or a gimmick, it has an emotional core that impacts how the player views it. mega evolution is a celebration of how so many fans really love the pokemon theyve raised.
z moves, while certainly very different in concept and execution, i think feel similar in that you actually have some kind of an emotional response to them. they're an important part of alolan culture that is treated with a great deal of respect by the people there. its seen as a huge deal to receive one, more so than something like a gym badge would be. that's why you have to earn them by defeating trial captains and the island kahuna.
meanwhile, dynamax and terrastilizing feel so stale in comparison. sure they're cool but, there's just not as much flavor to them. dynamaxing feels especially dull, as the game is very hand holdy about where it wants you to use it (literally you cant use it anywhere outside of specific story battles and max raid battles) and simultaneously lasts too long and not long enough. it isnt balanced enough to last until the pokemon faints like mega evolution or terrastilizing, so they made it last three turns instead, but it lacks the bombastic energy and visual flair of z moves to make such a short span of use feel significantly impactful on the player.
Some Pokemon fans really need a sense of humor. I'm sorry but after years of people wanting dunsparce to get an epic evolution only for it to evolve into a slightly longer version of itself and its called fucking dundunsparce is the funniest shit ever and y'all won't admit it