All Names Taken -
I wholly agree with your point concerning the risk of investing with any one specific airline customer in mind. MAN has bitter experience of this having designed a new terminal around the purported requirements of British Airways a couple of decades ago.
But ultimately airports must plan sections of the complex around the different niches of its largest customers, whether that be Jet2 with it's dedicated check-in hall, FlyBe with a domestic hubbing lounge, or facilities designed for EasyJet, Thomas Cook, Monarch, Thomson etc.
The business model adopted by the no-frills players is so widely copied now that I don't believe a dedicated no-frills facility at MAN would be particularly high-risk. Yes, one company could suddenly withdraw. But no-frills is an active marketplace and other carriers would consider taking on a functional business space subject to attractively-pitched fees. Look at the no-frills pier at AMS: it is rammed and other carriers are fighting to get space in there.
Whilst a suitably-motivated Ryanair could justify occupancy of Pier B on its own, I suggest that a dedicated no-frills pier at MAN would never run short of business in their absence. MAN is surrounded by a hinterland densely populated by customers with a love of leisure travel and an addiction to keenly priced airline-tickets. The names of the travel providers may evolve over time, but the core market for the no-frills product is established and growing. The high-end niche is more vulnerable than no-frills.
Ryanair does have leverage over MAG. They could throw a strop and suddenly withdraw at some point in the future. But leverage works both ways. MAG owns and operates the airport which controls the vast bulk of Ryanair's access to the lucrative London market. MAG owns the infrastructure at Ryanair's biggest base by far. It is in the interests of both companies to play nicely together. And with former Ryanair executives occupying key posts at MAG, the opportunity to understand respective business needs and pursue mutually-beneficial goals has never been better.