I like playing generational style the best, and so culling has made me hesitant to really get into Sims 4 because I want my family tree recorded, and never culled. When I had my legacy family and the tree started getting culled, that game and style of playing was 100% ruined for me, no lie.
They need to disassociate the family tree from being directly reliant upon sims existing in the world (as people or as ghosts) in order to show up on the tree. Ancestry (family trees) should simply be a hard record and not change for as long as that save exists, regardless of culling.