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 to leave if they say they want to, a football club should not be built around a few players but the entire team.
The departure of Sadio Mane broke a potent striking link that was a terror to any club defense in the world. Mo Salah and the Senegalese striker understood each other so much that it was easy to get goals with less effort. But Liverpool began to recover towards the end of the season and were able to at least achieve a Europa League slot. Clubs should also groom younger players that will replace stars player because they can leave at any time. Players should also be allowed to leave because forcing them to stay will make them ineffective.
Man United have wanted to keep David De Gea for next season, but the contract and salary deals are the main obstacles and demanding to be the first choice in the Man United squad cannot be given by Ten Hag. Because Ten Hag prefers goalkeepers who can build attacks from the back, which is why currently Onana is targeted to replace De Gea. It's only natural that Ten Hag did this because every coach has his own vision and mission in building a squad and so does he.
David De Gea was one of the highest-paid goalkeepers in Europe. But his performance has reduced drastically in recent times, yet he still wants the same wage. Manchester United is unwilling to grant him such a lucrative contract. Andree Onana will be a better and cheaper alternative. He is still young and will spend more time with the Red Devils. If the deal to bring Onana fails the club might consider bringing back the loaned Dean Henderson from Nottingham Forest and also put much effort to sign the Japanese international goalkeeper Zion Suzuki.