The Shimotsuki clan existed before Ryuma. It could very well be that Ryuma never had children and the current Shimotsukis are from a different branch.
If Ryuma was anything like Zoro, I can't see him having children.
I don't think so, because Ushimaru is called a direct descendant of Ryuma.
Either way, it doesn't matter. The dude lived like, what, 700 years ago? And he's the only one in the entire history of the family who rose to global fame and greatness.
Anyone who thinks this lineage shit means anything for Zoro's strength/talent/dream is hard coping with the fact that Zoro didn't get shit from his parents and is self-made unlike other characters in the story.
With that being said, Oda is trash for putting this stuff in the SBS and allocating god knows how much panel-time to fools like Momo and the scabbards. Zoro is simply one of Oda's merch-selling tools.