An airport is a strawman argument. The issues are there fold. Infrastructure, quality of life and jobs. Infrastructure would make effect the other two.
Better road and rail.link will bring London and with it investment opportunities. When the discrepancy between jobs / wages and affordable quality housing is narrowed students will be more likely to hang around if businesses can be persuaded to invest.
If there was a motorway to Exeter the airport would be far closer. I know people who pretend to live in London and use the sleeper train from Cornwall for next day pitches for consultancy / contracting / freelance work. If that journey was drastically reduced Rhodes companies could be based in Devon and Cornwall without pretending.
Nothing will happen until there is a motorway and a sub 3 hour rail journey to London.
At the moment Plymouth City Council and Cornwall Council don't lobby central govt enough.
What does Plymouth need? To be taken seriously. It needs to lobby Westminster