IF the service was frequent and reliable I see no problem, St Erth > Hayle is at worst 10 mins along the causeway so I think it quite realistic, especially if the number of available onward rail services at St Erth was significantly higher than Hayle.
More frequent than the one train every 12 hours currently stopping at Melksham?
There are 15 trains one way and 13 the other calling at Hayle in the current timetable, with a population of just under 9,000 and a footfall of 60174 journeys per annum ticketed to or from there. That's about 7800 trains per annum calling, each picking up / dropping off 8 passengers, and just under 7 journeys per head of population for the year.
Continuing Phil's Melksham comparision, 2 trains call each way for a population of around 22,000 and the ticket sales were 38081 for the most recent year for which we have figures. That's around 1400 trains, each picking up / dropping off 27 passengers if the figures are to be believed (I don't), and between 1 and 2 journeys per head of population for the year.
I have just been up to one of our alternative railheads, Chippenham, to pick up a customer who arrived there at a time that didn't connect (in his opinion) with the only piece of public transport that links the towns on a Sunday - the 18:47 train. The round trip took me about 50 minutes including wait time at Chippehnam, and it involved me going right through the town centre twice.
I'm always looking for comparisons
and I know I sorta started this one but actually it doesn't work terribly well. Although passenger numbers Hayle -> Melksham are in the same ballpark, so much else is different. 7:1 ratio on trains calling, 3:1 ratio on population, 5:1 ratio on bus time ...
Critically, Melksham's passenger numbers is supressed by having just a skeleton service that gives no realistic round journey opportunities, and I suspect that's not a limiting factor at Hayle although the 7 journeys per head of population figure is a very low one to what we see at other stations in our neck of the woods.