If PLH can be reborn for everything but CA, then good luck to them, but our experience here suggests it is needed, not just because of the handling fees, but also due to increase fuel uplift, and the major add-ons like parking, even a hotel was talked of here (now getting nearby as part of other development).
As for commercial? I seriously think you are pushing it, but I wish you lujck.
Two big killers - firstly, double dip APD really hurts in the pockets of the discretionary travellers you need to fill the seats, and this is before you consider an additional user fee (NQY style).
Secondly, as groundhog has already pointed out, the train is always going to win hands down on frequency, and sheer number of seats.
Comparing walk up rail fares, used by 2% of pax, with air is simply not telling the whole picture. It is also untrue to say you won't get a seat. There are numerous ways regular rail passengers will use to get a seat virtually every time, and for the higher yielding pax you'd need to attract, this includes going first class, aswell as reserving ahead. Not to forget that most trains to London get busier as they approach the capital - so board at Plymouth and you should get a seat, in the unlikely event you don't, stand in the middle of the carriage until Exeter where there is always some offload.
Given the way the modern aviation system works, most people will prefer to be treated like cattle on a busy train, knowing they can turn up most times of day, rather than go through airport screening for a cattle class airline seat on flights which realistically won't be operating more than 3x daily rotations.
total is 2h32m, compared to an average of 3h20m by train
That really is a very small difference. People tend to lock in to the fastest times, in this case 3h dead on, so even if the distance is less, we're talking similar times to Paris-Marseille, where the train still wins the lion's share of the market, despite going against a shuttle service that offers more departures in an hour than this route will in a day, and which offers global connections through CDG.
Viable have the figures, the aviation experts who've been involved in aviation for years, both in Plymouth and Worldwide.
And who have been brought on to make the case, so they'll play the best hand. I suggest that between the various commentators on this thread, there is more than enough expertise to say that PLH as a commercial airport is distinctly un-VIABLE.
The successful route was NQY to/from LHR operated by a HPR7 with average load factors of some 70%
Look how even NQY is struggling today, despite being up against a FAR worse rail service. What yields are the airlines getting at NQY? Is NQY even really viable?
One final thing to remember if you don't want to take what myself and others here are saying. Take a trip into Paddington, then pop over to Euston. What difference do you notice between the two train types - (not the superficial difference in age / traction)?
Now come back and tell me that yields for a PLH to London Parkway Airport service will still be viable.