How do we know he won't refuse to be sold?
PSG could be talking to Raiola right now, and he tells them save your £50m and hold off for 5 months.
Pogba will sign on a free and you pay him £30m direct.
There's a huge risk involved in someone who will be 29 by the time their contract finishes, planning to leave on a free.
One big injury and his 4 year contract at £400k a week with a £20m signing on bonus turns into a third of that.
Would he risk a £80m contract guaranteed and on the table now (with a fee for United) for a £100m contract on the table in 12 months that could end up as very little with an injury?
In addition for the next 12 months he would be losing the difference between what his United salary is now and what his PSG salary would be (this might be £6m).
I think the human aspect is underappreciated as well - once you've decided on the next step of your career, most people are keen to get on with it, rather than waiting. He may think he's potentially missing a Champions League trophy, as well as domestic honours too by waiting.
Finally if he stays there's a risk of Ole using him sparingly. Why play a player who won't be particularly committed and who won't contribute long term when you have Fernandes, Rashford, Cavani, Greenwood, Sancho, Martial all vying for the same attacking positions? With the World Cup next Summer he will want to go into the tournament in good form and fully match fit.
It would have been easy for Varane, a player in a very similar situation, to wait for 12 months for a much bigger contract; however I'm sure for some or all of these reasons he opted to move this Summer.