I understand the reliability hasn't been great, which is the case with all the 'proper' new fleets it seems. A follow-on order would presumably iterate the design as lessons have been learnt.
Refurbishing existing sprinters would be good and well but still be the same number of coaches as before whereas D-Train might be four or more cars. You would also be stuck with the poor air quality and energy efficiency that is the result of using a 1980s drive chain.
Wouldn’t it be simpler to stick a new motor and drive train under something built for such things rather than an electric tube train??