I think Politano's decline last season was more about the whole team being a total mess and not in coach's control at all.
Politano can be a starter at top4 level. Not top2 where Inter is at. Skillful but also pretty weak player, can be easily pushed aside if you can catch him. If he trains his shooting and goalscoring, I'd bet he could be even more. And with training I mean Zlatan-level training. Zlatan was pretty atrocious goalscorer when he came to Italy, started to train for real at Juve and the rest is history. He was never a natural goalscorer but trained himself to still become a hell of a goalscorer. I see the requirement for Politano to step to the next level is to start training super hard. Obviously at Inter his future was completely blocked because of Conte and his system, so with this move Politano can get his career back on track.
And to make it clear, I'm not expecting Politano to become a great goalscorer ever but with hard training he could become a very good or at least very useful player if he can step up is goalscoring ability a notch or two.