Walking away with a pocket full of dosh as you put it was only an option should the airport be 'unviable'. Until that time they were not entitled to benefit from such an option.
It's not that they didn't have an interest in operating Air Southwest, that is not my point or assertion - for much of the carriers existence it made money, so Sutton made money, and everybody was happy. But in the first place, the prospect of what was then dubbed 'an airline for Plymouth' pulled certain levers in the political world which saw Sutton awarded the said lease and terms.