Who said anything about him being happy sitting on the bench? I'm just assuming that if we were to sign him, it would be as a rotation option, not as our main attacking signing.
If we're signing 29 y/o late bloomers with one truly great season behind them to be the feature of our attack, we have a long way to go to get back to where we want to be. If on the other hand, we were to sign someone like Mertens/Perisic/Costa in addition to someone like Griezmann, I'd be all for that. We could do with a versatile, experienced attacker with a bit of pace in the squad.