I definitely agree with you here that it's a bad move developmentally speaking. But, the more I thought about it, the more I think it has less to do with tennis, and maybe more to do with finances.
As of the end of this yr, he still didn't have a racket sponsor, and he's no longer under contract with Nike, so that's what I think was the reason for the split. A full time coach is gonna be separate room & board on the road, plus travel expenses, plus probably a base salary & a % of winnings, along with maybe bonuses for certain milestones reached.
Just read that the reason DY went back to being coached by his Mom is b/c the USTA asked him to come down to the Boca or Carson training ctrs w/o his mom, & he refused, once again.
Really stubborn. I think it's a bad move on his parents part also. They should be encouraging him to be independent. If things dont work out he can always go back to his Mom as coach. A couple of weeks in the off season w/o his Mom isnt gonna kill him.