Jose is dead set on investment being the cure-all, and he's not wrong. He had his Porto moment last season, but the Portuguese league didn't have Manchester City's billions and Pep Guardiola in his second year.
He has left all of his previous clubs when he felt the ceiling was reached. Where he couldn't eke any more from his team, and the clubs' investment couldn't get any more ambitious. He probably feels he's gotten the maximum from the team, but United's investment plans have a lot more potential (being the richest club in the world), hence his continued fight.
So whether you're Mourinho in or out, depends if you think another coach can get a higher maximum from the team in terms of results/playing style, or if you actually like his personality, believe in the romance of long term management hire and are willing to wait for the board to green light more investment.
Both sides have a compelling risk/rewards.