Again, false information repeated again and again, without verification, and people believe it. Verratti has not renew, period ! So he can't have been forced to renew, indeed.
Verratti had the shittiest agent in the world that was renewing every year for tiny increase. This is why he had always 5 year contract and still low salary in relation to his importance in the team. Now that he is with Raiola, he still has not renewed and i supposed he will milk the situation.
Look :
https://www.transfermarkt.co.uk/marco-verratti/profil/spieler/102558
I always find funny when people attribute crazy things to PSG they wouldn't believe for another clubs and that becomes an universal truth somehow. All PSG did was telling him, he has a long contract so he is not going anywhere and since his agent had been very disrespectful, to fire him, which he did.
Coman, Zagadou and Rabiot are totally different issues because they are acadmy players. It is more like Fabregas issue for one and Thiago Alcantara for the other.
Coman and Zagadou went fot clubs known for poaching young and free players like Juve and Dortmund. PSG had offered them great economic package and solid role in the first team as well. In the case of Coman though, coach fucked up playing Menez who was already out of contract soon instead of him and he got angered (Thiago situation). Zagadou left because Dortmund and Leipzig pays a lot to steal young academy players. France rules don't permit to match those offer on very young player and their contracts can't be longer than 3 years (they sign it at 15 most of the time). So, by the time the player can really have a sight to the first team training, they leave for the money.
Rabiot is a totally different case and you already know everything there is to know about him.
About the second part, i would ask you a question : Why would you buy a highly rated prospect, for an almost record fee, when you could buy an already established player in his prime (let's say 27/28 y.o) around the same price ? Because you think 1) his ceiling is maybe higher, 2) he can set you for more years than the other guy and 3) you have resell value at the worst case.
Now, if you know the guy doesn't see himself staying longer than 5 years, the second point is void and the first one as well because you'll not benefit from his higher ceiling that he will fulfill in his new team. All you'll have gained will be his resell value. At the same time, the established player in his prime would have maybe given more for a good CL push.