In June we had two pleasant weeks staying in St Ives and fully appreciate what you are saying about rail travel in Cornwall. We travelled down and back by train (just under 324 miles each way) and there was no shortage of other (non-locals) on the trains. Whilst in Cornwall we travelled around, both on bus and train, and were surprised at the low fares down there.
I do understand your problem with getting around in Cornwall by train. I planned to visit Looe but the journey was just too difficult: the core mainline service is OK, - one through train to Paddington or Cardiff and a slow all stops to Plymouth. However for getting onto or off the branches (except from the St Ives and Falmouth lines) is all wrong. The timetable is constructed so that the mainline acts as a feeder for the branches going westwards and the branches feed the through trains going eastwards. That means that taking a westward train to Liskeard get there just after the branch line train departs. similarly, coming back up the branch, the train gets there to pick up passengers that have been dropped of the mainline, but the mainline train is long gone and iot's nearly an hour to the next one.
The buses were OK except that on some of the hourly seervices, there would be missing ones. For us it was the T2 from Redruth back to St Ives, there was an extra hour to waste.
However, lovely county and the people were fine, - can't say the same for all the visitors!