If the problem is only to play in the Champions League, basically PSG will also play in the Champions League. But the problem is not about playing in the Champions League but about awards and trophies, because if Mbappe continues to stay at PSG then his career will probably not be as bright as Haaland's. Therefore, even if PSG hold Mbappe in reality it will only be useless, because in the end Mbappe will not be willing to extend his contract either. Thus, of course, it would be better for PSG to be ready to let him go and if he is linked with Real Madrid then of course it is the very right club for Mbappe.
Current trophies achievement seems Haaland is better than Mbappe, winning Champion League for the first season with Manchester City made Haaland has better performance in this season from Mbappe. I think right decision for accepting Real Madrid offering for Mbappe and has bigger chance for winning Champion League tittle in next season with Real Madrid than stay for PSG.
Madrid attacking line position still empty after leaving by Karim Benzema, I sure Mbappe is the best choose for filling Benzema position in Real Madrid and actually Mbappe has chance winning many trophies if play for Real Madrid next season than keep extend one year left for PSG before contract over two years later.