In my mind this is the same question as “why will people run email or web servers if there is no incentive to do so”. They are pretty useful for communicating and coordinating things on the internet in a store-and-forward way with a data format that is simple and extensible. I would say thats more than enough reason to run them.
Discussion
So what I understand is that, given enough user traction, eventually relays will have to transition to a paid version or some sort hybrid (offering tiers for example) in order to sustain themselves, thus making this the incentive to run them.
Relays can be sustained by the business model of the application that is using it as a backend.