The ROSCOs could buy stock now but they won't do without the guarnatee from the DfT that it will be used. The franchises only last 15 years so even if a TOC contracted to lease the stock for the entire term of the franchise this does not guarantee that the stock will be used full its entire life.
You're right to say that the ROSCOs could buy stock now but dont due to the lack of DfT guarantee the stock would have, however if *no* stock had such a guarantee then that issue would be removed.
Regardless of franchise lengths the ROSCOs would know that the vast majority of the services will continue so stock will be needed. It could even be signed for longer than the franchise length, in a similar fashion to the facility charge for Evergreen 3 which is attached to the Chiltern franchise.
There really is no need for the DfT to be stifling the ROSCOs procurement strategies with red tape.