If you start letting any player...let alone Ronaldo...out of contracts you cant sell so they can move on for free anywhere you are opening a massive pandoras box for years to come. I would actually imagine other clubs would be pissed at the precedent.
No matter how great it would be to get rid of the issue...we need to either sell or keep him.
So if we sell him for 2M £ it makes any difference?
Or do we need 6M £?
The club and others couldn't care less if there is a fee or not. Free releases are quite common, especially if you have players with over valued wages. They turn into zero or even negative value because of their salary.
Europe is full of formerly known players sitting their contracts out on some bench. Every league has a few of those overpaid backup goalies still at their clubs.
They all got the chance to determine their contracts but they don't want to because they rather make more money than play more minutes.
Letting Ronaldo go would in no way set a precedent, the only reason it does not happen too often is because the players won't agree. I'm sure Jones, Bailly, Romero or Lingard all had the offer to nullify their contracts at one time.
Ronaldos salary costs the club over 20M a year.
Letting him go now means losing a very hypothetical fee which no club realistically would pay. And we already knew that last summer, so those 15M were not invested in hopefor profit on that.