Lufthansa tried a 50 seater 'pocket rocket' operation thru BRS, with all of BRS's catchment area, it lasted one season.
PF
The BRS-FRA route was actually operated by a Eurowings Bae 146-300 and carried just shy of 100,000 passengers in its 13 months of operation but apparently not sufficient to satisfy LH, and it was axed as the recession began to bite in the spring of 2009. Perhaps a 'pocket rocket' would have been viable - BA franchises operated the route for many years using one.
But your point is well made that if the BRS catchment could not sustain this type of operation the smaller catchment of Devon/Cornwall would be unlikey to do so.