Being that the challal status (resulting from a kohen having children with a woman that is assur to him mishum kehuna) applies to all of his descendants for all of eternity and such people (the daughters) can never marry a kohen, even if there were only small number of challalim hundreds of years ago they would still have thousands of descendants today who would be assur to marry a kohen, yet I have never heard of anyone who is a challal. Any ideas?
Your premise that the children of a Challal are Assur le’Kohen is incorrect. See Shulchan Aruch Even HaEzer 7 16.
Well, he’s only partially incorrect. Any daughters of male-line descendants of challalim are forbidden to Kohanim. So, the OP’s question is still valid.
It’s a B’feirush Gemara in Yevamos that we have a Havtacha from HKBH that a Mamzer that we don’t know about won’t have viable progeny (past grandkids).