This is an interesting question. I have witnessed the behavior you describe several times, and I have seen them hold their own for crazy long times (much more rare), and I
once witnessed Tyal rebeling from their Khuzait conquerers and shortly thereafter joining the Nords, giving them an extra clan they normally do not have.
I like that there are different possibilities. Based on my few playthroughs so far I like the randomness of it all. Just throwing in my experience to say that
it is possible that they can join a kingdom, if they do so while they still hold the fief, at least. I would like it if it were possible for them to start a new kingdom of their own, have others join them, etc. I don't think that is a current possibility.
I was never able to get such a clan to join
my kingdom once they were wandering around, fiefless and enemies of everyone else but me