Couple of points occur to me:
- Critch may think he's taken us as far as he can, promotion and above expectations finish in the Championship, the next season could be pure downside, he can't exceed expectations, he might fail to meet them and end up sacked, in which case all the reputation he has established so far is wasted;
- Critch doesn't have any experience in coaching PL players, it's a big gap on his CV, this move should allow him to plug that gap.
So a backwards move or not, I think it may make sense for his career, and it's quite possible he'll get a shot at the top job at Villa, either way I expect to see him back in management in a couple of years or so.