So that's Sane, Stones, Bernardo, Mahrez, Rodri, Cancelo, and now Ake who came in, didn't really look all that, and then suddenly improved in their second season (or after a while).
And I'd say Grealish's overall performances have been better than Rodri, Cancelo, Mahrez and Bernardo's were in each of their first seasons, so there's evidence that he'll get even better.
As always with City and any new signing we make, they get about 18 months to show that they can be of value to the team. If not, they get loaned out or sold. Grealish will either improve or leave. What's the biggie?