He wouldn't be a 'massive' upgrade at all. He's better at some things, not at others, and I'd argue he's not as suited to being a squad player for several positions as Mount is. He's certainly got more flair than Mount, but he's far more of an individual than a team player.
And no 27 is not that old, but his contract runs until 2027. There were rumours there's a release clause but it's meant to be at least £60m. He'd want big wages and a 5 year contract, minimum. You're signing him at a point when he might already start to lose a step, especially if he has any more muscle injuries.
It would just be a pointless signing. Basically like signing Mount again. Good player, but doesn't really fit anywhere. Better off spending the money on a new LB and upgrading CM.
Olise was the player we should have gone for, but that ship has sailed.