A 31 year old Palhinha, which is what he will be before the next season starts, at 25 million isn't cheap. He would have virtually no resale value, and a few years of him tackling people at a decreasing efficiency due to aging wouldn't be worth all that.
Hell, If Levy had sanctioned some equivalent of such a move, it would have been crucified as another instance of this overblown notion of Levynomics. I'm shocked at the suggestion to be honest.
If Bentancur is actually staying, making this permanent would only take minutes away from Gray while ensuring that we won't sign a DM. Be careful what you wish for.