With franchising now dead, any decision on whether it should continue of not will be in the hands of the DfT» (or their successor in the form of some kind of national rail body that's being mooted) and within the management contracts they prescribe.
But the sleeper was a "franchise" requirement - in the
SLC▸ - so it always was a DfT decision. That may have resulted from discussions with the
TOC▸ , but no doubt the controller within DfT (of whatever shape) is still likely to consult within the industry before setting the service for future contracts.