What choice does he have, he signed for the club before Chelsea got into all the problem started facing before the season ended. Christopher Nkunku didn't sign in this summer transfer window, he was already a Chelsea player bought during the January transfer window (that's if I'm not mistaking) and was only wanting for the season to end before he makes the transfer to Stamford bridge.
Yep. It may be too early for him to decide joining Chelsea. When Chelsea ended up the season with a bad result, he has no chance to change his decision because he is already a part of Chelsea squad. However, if he can play well in Chelsea, there should be many offers from other clubs. Or if he can help Chelsea to perform better, Chelsea may have a better position in EPL league in the next season. There should be a chance for him to have a good career in EPL, Chelsea also is rebuilding their squad.
He's a good player and with the new coach (Pochettino) coming and possiblity of signing new players, they'll definitely enjoy their time playing for Chelsea. We mightn't be in any big competition but that gives the players a higher chance of winning a cup trophy in England and giving the fight for the title a higher possiblity of winning it. Chelsea has nothing to lose next season so they should be giving their 100% for the title challenge. Chelsea won't be playing under any pressure and it'll give the new signings enough time to adopt to the league so he's still making a good choice coming to Chelsea.
Sure, Nkunku is a great player and Pochettino may improve Cheslea performance in the next season. However, I'm not sure Chelsea will target for winning EPL trophy in the next season. They may target a realistic goal, they probably want to return their position in the top 4 (UCL zone). I think it is Chelsea goal for the next season, but it won't be easy since EPL becomes more competitive.