My guess would be, the lack of 3rd party renderers could be a mixture of modo being "the new kid on the block" and its default renderer being good to a point, where it could be considered serious competition for most other renderers (considering it comes with modo for free and the level of integration). As for user base and licencing, i can't imagine any problems there, there's a lot of 3rd party stuff (not rendering related) popping up for modo and the users seem to be buying it. About the technical aspect of the implementation, I can't say anything on that, being programming-illiterate, but the foundry boasts about a great API for modo, so I have no reason for not believing them, at least until I hear otherwise from someone who understands this stuff better than I do.
Either way, the decision has been made, so i'll keep hoping for some skilled programmer to come along in near future to help with modo integration.