You can't use the first sentence as a run-on and not see the contradiction, surely? Every human on the planet gets better by doing, repetitively and expediently until they are determined masters of a skill/craft. By removing the opportunities to even explore that element of his game, Grealish is reduced - or marginalised - as a specialist with a specific job to do in the team, one he's the best in the entire league at, and very probably the reason he was targeted by City in the first place, but a specific job nonetheless.
Your posts highlight it - get from point A to point B and then pass it to someone better than you are at the next phase. This can be considered optimisation, because, in a way, it is. But it can also be considered marginalisation because there's a glass ceiling to what Grealish is allowed to do and what is frowned upon because his attempt from that point onward can be taken on by - in your own words - someone better than him in the next third of the pitch.
This can be seen as a matter of perspective - is Grealish better or worse for being optimised? By removing all form of expression, but having him run like a battery at what he is exceptional at, is he a better or worse player? An underutilised one, or not? If we were using American Football terminology, Grealish is/was a sure thing as a yard gainer, which is an invaluable asset for any team, not just Guardiola's. Having a player that 9/10 times can gain you 30+ yards is a cheat code in itself for the sheer consistency of it. In the minds of those who went out and got what Guardiola asked for (some form of yard gainer, I'm sure), City were provided with an asset that really allowed and afforded them to play in a different way, that, in part, was less team-based, as Grealish could gain those umpteen yards by himself, instead of by how City used to progress those same yards as a unit, which takes time, effort and a lot more pieces optimally in place to accomplish.
Guardiola has said himself that ball carriers and dribblers are the cheat code for what they enable a team to do in a fraction of the time, and in his world, it makes perfect sense to have used Grealish as he did, with him passing the baton off when he got to the - for him - hard yards. At other clubs, under managers who don't see the game in such a binary fashion, the "Villa Grealish" goes on to a bigger club and isn't anywhere near as efficient as Guardiola made him, but becomes a better version of the player he got to that club as. Better decision making; better understanding of the game; better understanding of self and when and what to do in certain positions/situations via the aforementioned honed and refined practice, etc. etc.
As I've said to you before, Grealish is the other side of the coin of system cog verses expression; he and Foden, both, on opposite ends of the spectrum, which makes them perfect case studies. One cannot determine whether the Grealish we got to see is better/worse than the one who made his name. We can say that Guardiola brought him success and made him an important and essential cog. But it can be said with utmost certainty that Guardiola made him boring, predictable and perfunctory - do what you're good at, and don't dare overstep your mark. It's the very definition of expressionism lost, even if we understand why and the [clear & vital] benefit of it.
Through the eyes of a fan of flamboyance and expression, Guardiola getting his hands on this type of player - one he deems exceptional for one thing, but not good enough for the other - it's got to be seen as a loss "to the game" because one stem of free-flowing expressionism has been lost, and another Grealish then has to take on the mantle because the one that was there no longer has his. The problem here is, Grealish's don't come around often, which is why it's going to be seen as sad when one is lost to the system despite the success it brought the club who purchased him.