Liverpool experienced a decline in form because Mane left, as well as when Ronaldo left Manchester United to join Real Madrid.
Good players will definitely be missed by their erstwhile club, but if the team is made up of quality players, their absence will not be felt that much or for a long period of time. Liverpool didn't have a bad season because of Sadio Mane's exit, they had a terrible season because most of their players were in horrendous form and the manager wasn't able to bring the best out of them. I think clubs should always allow their players leave if they say they want to, a football club should not be built around a few players but the entire team.
Agree, I also don't think Liverpool's decline in performance last season was due to Mane's departure. Because after all, even though for example Mane still played at Liverpool last season too, Liverpool's performance could still drop, and we won't know what will happen for sure either. Because I believe, at least every team will experience a decline in performance but because Liverpool's decline in performance occurred after Mane left, then of course most of them will think that losing Mane greatly affected Liverpool's performance. Even though in fact it may not be at all, because in the end as time goes by Liverpool's performance begins to improve and as for the results and performance next season, whether that has an effect also because of Firminho's departure, it is interesting to wait.