If Cristiano Ronaldo finally moves to PSG, it is likely that Kylian Mbappe will be released by PSG and potentially join Real Madrid, and I think this is a win-win solution for PSG, Mbappe, and Real Madrid.
But there could be a surprise if it turns out that Real Madrid brought Ronaldo back to Santiago Bernabeu. Even though Ronaldo is no longer young but he can still contribute a lot to Real Madrid (at least for the next 1-2 years) and of course he will adapt quickly because he has played at Real Madrid for a long time. Other than that, to sign Ronaldo will not be as expensive as Kylian Mbappe, so Real Madrid can save money to buy another player (because Real Madrid really needs a center-back).
How is this a win for PSG? They are not getting to keep one of the best players at that age, dude is a player that can provide you with top notch quality for 10+ seasons and you are losing him and getting Ronaldo that will play maybe 3-4 more seasons which I doubt will even happen, but if he does than there is nothing impossible for Mbappe to provide nearly 16-17 seasons as well, so you are literally losing time here.
Ronaldo could be seen equal to Mbappe right now by some people but I also do not believe that, Mbappe is a better player if you ask me and that is currently of course, Ronaldos peak was unseen, only messi was there. So a trio of Ronaldo,Neymar,Messi at the top and other players backing them up such as Ramos, Donnaruma, Wijnaldum just the new names and many other, I would say they would be great, but Mbappe is still better choice. They are forced to do that, not their preference.