I'm all for enhancing the capacity on the Newquay Branch to accomodate long-distance services and local trains connecting to the main-line, but it is hard to see the benefits of through running to Truro and Falmouth, and easy to see the operational disbenefits. Very few punters are going to use it from Newquay to Truro when the rail distance via Par is nearly 40 miles; by road it is just over 13. The rail journey time will be around 85-90mins; by car 30mins and by (slow) bus 57mins. I'm sure the fabled 'electric bus', so often proposed for Tavistock on this board, would be somewhat quicker. St. Austell isn't much better either; the rail distance via Par is nearly 26 miles; by road 16.5.
As has been mentioned, the real 'levelling-up' agenda (oops - we're not allowed to say that now) would be to upgrade the Parkandillack line to passenger standards and reinstate the missing link to St. Dennis Jcn. The existing line to Goonbarrow Jcn could be abandoned, and Goonbarrow Jcn to St. Blazey retained just for the (dwindling) clay traffic. I don't buy the supposed SSI problem for Goss Moor - the reinstated line would be on the very edge of the moor and is a former right of way - unlike the original A30 scheme for a four-lane highway right across it. The rail distance from Newquay to St. Austell by this route is about 17 miles, and actually goes, you know, where the punters want to go. There could also be intermediate stations for the (deprived) clay villages, and there is room at St. Austell for a third platform on the site of the old Motorail facilities.
I'm old enough to remember in the 1970s when the Newquay trains were briefly extended to Bodmin Road (as it was then) , in attempt to cut out some intermediate stops and speed up services on the Cornish Main Line. It was quickly abandoned as the train miles accumulated - I hope I'm wrong, but I can see the same happening with this metro scheme as it fails to draw in the passengers.