Rails versus wings
Regarding weekend rail journeys. Have you checked whether "engineering" work, (used to be called track maintenance) is being carried out. I know of a friend who did exactly what you did, and found that the line was closed between Tiverton Parkway and Plymouth. This entailed a 45 minute "transfer" from train to bus and a 2 and a half hour bus journey as it had to go to Exeter, Teignmouth, Newton Abbot, and Totnes to drop pax off.
And, as a previous poster also mentioned, the huge subsidies from government make it possible only.. No private company would consider, nor get the backing to run a railway purely as a private enterprise.
In addition...
The time comparison between London and Penzance is fine, if, of course you live in London. For someone like myself, I would have to get to Paddington or Reading first, adding at least an hour onto the journey not to mention the additional costs of tickets/petrol/ etc...
Unfortunately, successive governements, Dr Beeching, and of course, the unions have done for the railways ever being a realistic transport alternative in this country.
Oh, and don't get me started on how much more "green" high speed rail links are....