Imo, it is something like this:
1. no matter how good Verratti is, with him in the team and with current players=Psg will never get nowhere near the CL title
2. selling Verratti, getting weaker, but still very good midfielder and getting any players from Pogba, Cr7, Bale, Benzema=would raise your chances to win a CL, especially if you buy a Cr7, Bale or Benzema
While Verratti is awesome, he alone can't do too much.
On the other hand, Cr7 or Bale alone could do much more.
Something like:
-- imagine Barca selling Iniesta or Xavi, get weaker, but still good player like Rakitic
-- and for money earned from Xavi/Iniesta plus some cash, you get Neymar
So, instead of having Xavi+Alexis, you can sell both, add some cash and have a weaker midfielder (Rakitic) and much better decisive player, like Neymar.
So, if Psg is considering selling Verratti, they are preparing some "big fish" transfer 100%.
Exactly