I do also think this is a good decision!!
Maria seems to be at her top in singles, she is still young but already has a lot of experience, the perfect combination! She should make use of that to get out the maximum of her career in singles. This season could be her best ever, maybe she can reach the top ten and maybe a GS semifinal! Playing singles AND doubles all the time would be too much for her..
Kiri has won many tournaments in doubles, an olympic medal and the Year End Championships..
only a GS is missing, and I would say she can try that again when she is a bit "older", in 5-6 years or so...