They'd have to re-do the Z-moves in the new engine that they use for Gen VIII anyway - a waste of time if they decide that they aren't happy with how it played out - so I'm not sure what you mean about the "lot of work" that goes into creating them.
And like I said, Mega Stones are quite different, because they create *new* pokemon. Many people are fans of the mega pokemon, rather than the regular form of the same pokemon. Dropping mega evolution would upset many people because their favourite pokemon have been removed from the games entirely. This simply isn't the case with Z-moves, which are just a game mechanic (which are routinely altered or dropped between gens).
I'm not asserting that Z-moves are certain to be Gen VII (or rather, Alola) exclusive... just that there's no reason to assume that they'll continue into the future generations. The whole reason why they prevent transfer of items between generations is to allow them to change the rules, remove items, and control access to things.
Actually, no it is the case with Z-moves, because you have to keep in mind that there would be back lash because your taking away something people consider important to their favorite pokemon. You have to keep in mind for some people the Z-moves really help some of the pokemon out and make them viable in the meta, as for there being no reason to assume that they'll continue into the future generations, no there, is no reason to assume that there won't be away to get them in future generations.
This not something that's just dropped, I'm sorry but realistically their not going to go out of their way to create 18 Z-crystal for each pokemon type and then take that further and make pokemon specific Z-crystals, and the specific pokemon Z-crystals are like signature moves to those pokemon so taking them away, would like taking away a pokemon's signature move and yes people would be ticked off and upset if you take away one of their signature moves form them.
I mean basically here is what your saying that GameFreak would go out of their way to specifically make a unique Z-move for each of the 18 types and then make some signature Z-moves for some of the pokemon and then go for the moves that don't do any damage figure out what benefit they will provide instead if you use them with a Z-crystal, and then just turn around in 8th gen and essentially throw all that work away. I'm sorry but that is not realistic, if it wasn't as a throw concept then we would get a very generic Z-move no matter what type we used they just have one single Z-move that is the same no matter what pokemon uses it or what types of moves are used with it.
I mean you say they have to re-do them in the New engine that they use for Gen VIII however, that's not the same as having to create them from scratch, considering they already would know what the Z-moves would look like and have a concept of them they would just have to translate that to the new engine pretty much like they do with the other moves and the pokemon.
At most we could get a case of where in gen 8 they don't introduce any new Z-moves, but their will most likely be a way to get the Z-crystals just like their will be a way to get the mega stones, and the items that change a pokemon's form and the signature items of pokemon. Like it or not Z-moves are new moves and if you at it every generation introduce new moves and every future generation has those moves, so there isn't any reason to think that there will be no way to get the Z-crystals in future generations, especially when they could easily just make them available at a battle facility or something else and have them available in the post-game.
To you Z-moves may not seem like that big of a deal, but to plenty of other pokemon especially in regards to signature Z-moves of their favorite pokemon to give it a signature Z-move and then stripping it away in future generations would be like taking away a Mega Evolution for a pokemon, for example let's take Mew, who got signature Z-move this generation when Mewtwo got 2 Mega Evolutions last generation a significant amount of people were disappointed that Mew didn't get anything while Mewtwo did, and then in 7th gen Mew got a signature Z-move which made plenty of people happy because Mew seemed to be finally getting some respect, and now what your suggesting would basically like GameFreak be like " haha you thought your favorite pokemon was finally getting something, joking this is only for Mews in gen 7 but it won't carry on to future gens" yeah that's going to spark some outrage.
You seem to be misunderstanding how important Z-moves are to people sure there are people that don't like Z-moves, but there are also plenty of people that do and looking at it if Z-moves weren't popular then they wouldn't really be going out of their way to create more in Ultra Sun and Ultra Moon. No matter how you look at it or try to argue it a the end of the day taking away a Z-move would be practically the same thing as taking away a Mega Evolution, because like it or not people do have their favorite Z-moves and people do like it the signature Z-crystal for the specific pokemon. So their would be significant backlash if the removed Mega Evolution.
To put it bluntly you can try to say oh but it's different from Mega Evolution, so it doesn't matter if it gets remove all you want, but at the end of the day it does matter because taking away Mew's, and Snorlax's and Pikachu's Signature Z-moves would create as much back lash if you took away Mewtwo's, Charizard's, and Sceptile's Mega Evolutions.
Any way let's just end there and agree to disagree, because look at the title of the tread this isn't really this place of this kind of topic, and feels like were getting off topic here.
To bring this back on to topic as to whether or not I think Ash going back to Kanto could hint at Kanto in USUM, It would be a nice surprise but I'm not sure if they will actually do it.