But, if for example Manchester City is proven to have committed an offense and severe punishment will be received such as relegating Manchester City, of course it can still happen if proven to have committed an offense. At least, cases like this have also happened in the past but in Serie A related to Juventus, in the past Juventus also received severe punishment after getting the scudetto but Juventus was actually relegated and the scudetto was given to Inter. Until finally, all their star players is leave and only loyal players who stayed at Juventus like Buffon and Del Piero. Therefore, any violation if properly proven then the punishment can also occur but yes, hopefully a bad case like does not happen to Manchester City. But, if Manchester City is proven to have committed an offense, then hopefully the punishment is not as severe like relegating Manchester City.
That is absolutely true what you said about Juventus Turin and it is still one of these rare cases where a top team from a top league gets a severe punishment. But if you think about Manchester City and their standing in the Premier League and in the global football world, do you think that authorities would pull off that move and relegate Manchester City into the Championship? This is just speculation but if I had to make a guess, I would say that they won't get relegated. It could still be some severe punishment (definitely financially), but I just can't imagine that they would go so far as to say that they need to leave the Premier League. Maybe they would be suspended from the UEFA competitions.