Actually I think (since recently) your manager level will stay the same until you go up to the next number level. So he has some time to get back up to the point where his manager number will change.
As Staggy Stag says, mostly the not wanting to invest in T drives some high level folks to a take-over; some people see the same teams in League again and again and want to see fresh opponents, and too, sometimes you just want to start over.