The point is not how good Messi is. It's how old he is.
It's potty, but nevertheless justifiable if you pay £200m for a player who you might have playing for you for 5 years and who you then sell for not much less than you paid. If PSG were to do that with Neymar and you work out the annualised cost, it's perhaps not that bad.
But if we paid say £200m for Messi, we'd get 2 good years out of him, 3 max. And then sell him at a huge loss.
It's a completely different proposition. As another poster said, 2 or 3 years ago yes no brainer. But now he's 30? No, not imo. Not unless we got him for a relatively low price.