The issue with reopening the line between Bere Alston and Tavistock is there is already an existing "essential" service from Plymouth to Gunnislake. This currently operates with sole track occupation north of St Budeaux, with unsignalled points at Bere Alston operated by the train's crew after the train has stopped at Bere Alston, prior to reversal.
It is the section north of St Budeaux that currently lacks capacity for extra services. Any additional train on the branch line to serve Tavistock will require signalling at Bere Alston, and for a half-hourly service, a passing loop (or loops) would be needed north of St Budeaux with further extra signalling.
This would all add to the capital and running costs of reinstating rail services to Tavistock. It clearly isn't a worthwhile use of limited financial resources to do this, even taking into account potential social and non-financial benefits, so there is no point in undertaking further studies on the proposal for the foreseeable future.