Or how about we stick to comparisons in this country shall we?
Buffets on, Kgx to Newark, London to Norwich, Kgx to Leeds, St Pancras to Nottingham, Euston to Birmingham, Aberdeen to Inverness to name just a very few examples.
And you give examples of 2 to 3 hour trips in France well journeys of 3 to 5 hours are a better comparison and London to Penzance is more intercity than inter urban..
Ok we’ll stick with this country then, Paddington to Penzance is a route often quiet out of holiday time/summer season in terms of through journeys London to Cornwall, hence why in BR days the buffet would come off at Plymouth.
In the summer it is very difficult to match capacity to demand, 4-5 hours standing or those extra 16 seats per train which equates to maybe 100 less standing people a day...
As for dedicating 9 cars to Cornwall the 9 car 802s were built as a batch for North Cotswolds and West Country, on the North Cotswolds the average commuter is far more interested in a seat.
You talk about comparisons in this country how about Waterloo to Bournemouth and Exeter, like western routes this are a mix of ‘Intercity’ and ‘Commuter’ traffic, not dissimilar to the run down to Exeter from Paddington. Indeed Bournemouth was the last London and South East route with buffets which are finally now going as the 444s are refurbed as passengers have asked for more seats instead...
Ideally we’d have the space for enough seats, a buffet etc but our constrained by length railways makes that impossible.