I don't think that a business case has been made for their premature replacement.
It may have seemed a good decision, but many of the issues have not been resolved, and will be left to TfN to find funding for.
I agree that no explicit business case for Pacer replacement was made with the coalition government overuling the civil service and banning bidders from using them.
However Arriva was happy to bid for the franchise under these terms and despite the costs of new/refurbished stock are willing to have their subsidy level cut by 70% by the franchise end. They give no impression of being other than delighted with getting rid of the Pacers.
What issues have not been resolved? As far as I can see in the Franchise Agreement, it is Arriva that takes the risk of delayed cascades. In those circumstances it would be normal commercial practice to pass some of the risk in turn onto their suppliers, the ROSCOs.