I see zero reason that Gruden would want to get rid of a former DPoY unless he realizes that the Raiders do not have money and paying Mack that type of money (assuming they even could with the move to Las Vegas) would strap the Raiders for future years. It's really easy to say they have the cap space or the money now, but they'd be, essentially, paying two quarterbacks and we've seen teams get over zealous in FA and screw themselves in future years with their cap (looking at you, Jacksonville). 2020 is gonna be a really big year for them in free agency.
And I'm very sure the Raiders FO DID want to keep Mack, the same way that I'm sure the Ravens wanted to keep players like Torey, Osemele, Wagner, etc. It just doesn't always work out that way.