If we're worried about the supplier now then we don't bu from them. If they suddenly change down the road (and most established ones don't) then the fallback is either "we don't need support anymore" or (more likely) we start looking for another system.
I've been on the other side of this. We sell a product into an established mature domain. We're the "newbie" on the block. Most of the sales we get in this space are from customers who are unhappy with their supplier. We offer better sales and service, and obviously a smooth transition from their existing data (which we import.)
Neither product is OSS - but even if it was that would be irrelevant to the user. (It would however have made our integration code a lot easier to write, so there is that...) A lot of the users we convert have "bought" their software. It's costing them nothing to use it. But they switch to us anyway (we have a subscription model) because our model can afford to fund full time support staff, whereas the sales model cannot.
So, I think this "choose another servicer" is more of a theoretical than practical feature for most OSS systems. Obviously there's really good support for the really big projects, but basically nothing from 99% of them...
