They begin to build up an infrastructure of crews, ground staff, management, offices, equipment, contracts, etc
Do they? Ryanair don't employ any ground staff or management at Bournemouth, they have no equipment there and their office comprises a room in a portakabin. Their crew would just be moved to another base, as they do with the rest of their crew.
All it would take for Ryanair to withdraw, and they threatened many times, would be for Servisair to start charging them the same price that they charge Thomson. With Servisair hemorrhaging money at Bournemouth, something has to give.