I remember times back in the 1970's/80's/90's when you could only travel on the Inter-City services if you where travelling as an example Penzance to Birmingham and if you where travelling to Penzance to Exeter, then you would have to travel on the BR local services. Now, I am not sure how you would do that with the travelling public in the 21st Century, whereby there is probably twice the amount of people that is wanting to travel between Penzance to Exeter by train, than there was back in the 20th Century.
Now the fact that you see the Waverley service is at least something, as many places like Brighton, have long since had XC services taken away from them, such that they have to travel by Thameslink, or Gatwick Express to London and travel across London to get a train to Birmingham. Yes, it is a quicker option than the XC services which used to go via Reading/Oxford route to Coventry and Birmingham, but you only had one train. Now, it is such with the amount of services with Thameslink, Gatwick Express and Southern local services that you would not be able to fit into any timetable any XC service to finish/start to/from Brighton. The other issue is that there was not really the demand there to be keeping the XC services from Brighton, as there is more of a demand for people to commute locally and to London. The same I believe is happening in Cornwall, with GWR upping the amount of services that they provide to enable people to better commute through Cornwall and Devon.
When I look at Real Time trains, you do have the following XC services from Penzance:
06:28 Service to Edingburgh
08:37 Service to Edingburgh
20:47 Service from Edingburgh
21:35 Service from Aberdeen
In between the above services is there is movements from and to Penzance T&RSMD.
It would be good if cities/towns such as Portsmouth, Brighton, Eastbourne, Ashford International, Folkstone, Dover, Norwich, Cleethorpes all had similar covering from XC. But it will never happen, as the requirement for local commuter services or services where people can travel to London is greater than people travelling to say Coventry or Birmingham.
Not sure how you would do it, but I do think that the Department of Transport should send a survey where they have mobile phone/email contact details for motorists each year to find out where they have travelled too in their vehicles each year. From this you would have a better idea where bus services and train services are needed both locally and nationally each year.