Well, there are also a lot of descendants of Imam al-Hasan (A) especially in places like Morocco. It is recorded to have been said that if Imam al-Husayn (A) was killed, there would be no grandson of the Prophet (S) alive on the earth anymore, in the sense of the son of the daughter of the Prophet (S). This is true, as Imam Hasan (A) had already been martyred by that time.
However the line of the Imamate went through Imam al-Sajjad (A) so the line of the imamate would have been cut off in that way.
Anyway, it is a good question and demonstrates excellent critical thinking skills!