I dunno, just to throw a thought into the works here; don't we use a lot of things as a crutch in order to be able to deal with some heavy shit in life (says the woman that also finds it near impossible to be alone)
I have used my situation as a crutch. I am willing to admit that. I am working of being alone though, not filling my life with more partners in order to not be. I have definitely used the stability I have with the partners I have as a trusting safe place to stand up on and get my balance. Not just in being alone, but many things. My tribe is most definitely who I come home to after being out there in the world. Does that make them a crutch? I think so... I have had need of them as a crutch... but a crutch is meant to be until one heals I think... as in crutches for a broken leg for instance.
I think if I were you and at your age I would just leave them be and not bank on this being a long haul kind of thing. I think I would realize they are using me as a crutch also and get about finding someone else who is also mono, or at least more experienced with poly. This person is doing their thing no? They need support and I would likely give it to them by just being there as much as I feel comfortable.
You should be doing you thing too though. You can use them as a crutch also and be entertained and amused and enjoy their company until someone more suitable comes along. Why not? I think we all "use" each other and we all need a "crutch" I really.