I think it is a reasonable fear when ending an long relationship, even if you are not into BDSM.
You could say it is harder for a BDSM person to find another compatible partner, there are far fewer of us than the mainstream population. On the other hand we search much more 'on target', among people of our own kind, and usually with much more focus on needs and much more communication. I think one evens out the other.
But the fear would be there, as with other relationships. Yes, I for one know this feeling.