Ederson has played for Manchester City for eight seasons, and Pep Guardiola himself still has considerable confidence in him, despite rumors suggesting Donnarumma is being groomed to replace him. If PSG wait until next season, Donnarumma's contract will expire, forcing them to let the best goalkeeper go as a free agent. This situation isn't beneficial for either party, and there may still be ongoing considerations regarding the release clause and how this situation will be handled to ensure neither side is disadvantaged.
If this Donnaruma deal could happen, they will do that no matter what happens to Ederson, they can even loan him out at the end day if they have to, because getting Donnaruma is very important for them, he is not an old goalkeeper, and he could play there for at least another 10 seasons and that is a very good profit for them. They may spend like maybe 50 or 60 million, but they will have a goalkeeper for at least 10 more years.
Think of it like new Ederson, even if not for 10 years, they could have one for 8 ears just like they had with Ederson. How awesome that would be for City, and that is the key point here. It is not just about Ederson vs Donnaruma, or Trafford vs Donnaruma, it is about getting an elite level goalkeeper for so long.