This is a good point and one of the reasons I'd really like to see us try for Deschamps. We need someone that can actually take us on to the next level, not just to where we were under MON. Deschamps took a Marseille team that other managers couldn't take past "serious challenger" and turned them into champions, and he looks like doing it again this year so it's not a one off.
As much as I'd like it, I don't see how Deschamps is a realistic target though - he's now managing a club thats regularly in the Champs League, so he's unlikely to want to test himself at a club like Villa who are still battling for a place in the CL. His stock has risen, and if he were to leave Marseille, it would be for one of the major European clubs (quite possibly Arsenal if Wenger leaves).