Do we know that for sure though? Some traits might carry on for a long long time in the Narutoverse. On the other hand, nothing says those traits can't be awakened again - they had to come from somewhere right?
However, if that is not the case, then it is inevitable anyway. Their children can't go on marrying each other. If they were to have kids, they would inherit those traits for sure, yes - but what then? It's a one-generation-solution.
And I don't think Karin has that many fans anyway. Not to mention that Naruto and Karin have barely even spoken to each other.