I think it's hard to admit that Neymar and Messi's performances have dropped, because if that's the case I'm sure PSG won't hesitate to sell players. In this case, it seems that Neymar is the only one who gets this treatment and also, some sources say the main problem is about the amount of salary so that Neymar is no longer needed by PSG for next season. Because for me, Neymar's performance is still good but if PSG no longer need him because of his declining performance then that is a good reason to go on loan or sell him. Just can't imagine them confidently making a long-term contract for Neymar with a lot of money, but after Mbappe got a new contract Neymar seems to be no longer respected.
I've said this before, Neymar already got paid huge amount among other clubs and players, if he want to join Chelsea because he can get better pay check, he wouldn't get higher than in PSG because Chelsea haven't paid his current players with the rate that PSG pay to Neymar. Not to mention Chelsea has been bought many players which is their financial isn't really have huge amount cash flow to paid super star player e.g. Neymar.
PSG seems too over about their decision to give some privilege to Mbappe.
Neymar's salary at PSG is 40 million per season if he moves to Chelsea how much will Neymar be able to pay later? I think Chelsea will find it difficult to do this salary because it is too big so PSG are still able to pay high salaries with several other players.
This is still a rumor because we don't know the sequel yet, I think in the future they will reconsider, and I saw several news stories with many different versions of this rumor.