I can think of several reasons that a manager would leave. Let's analyze regarding Unai...
1. More money: Certainly other clubs have more revenue and can afford to pay him more per year. However, I think our owners know what we have and will come up with the money to keep him, even if it means spending less on players. Also, you have to think of money over a career, not just money per year, which brings me to the next point...
2. Stability of job: Clubs like Bayern, Barcelona, and Real Madrid tend to chew up managers and spit them out quickly. Unai has already been through that with Arsenal and PSG. He could make more money at Villa because he will likely last longer at Villa.
3. Resources: No dispute here...other clubs can spend more money on players for him.
4. Control: I doubt that any big club is going to give him the kind of control he has now. Certainly not Barcelona or Bayern or Real Madrid.
So, on the whole? I see him staying.