I think with them beginning production on Sims 5, and moving a lot of the Sims 4 staff over to the new game (nothing is confirmed i don't think but there's been enough job postings and leaks by now to assume that's where the staff that said bye-bye on twitter went) that the people that were apart of the stuff pack production team probably got moved to game and expansion teams to fill in the empty roles. So I'd guess stuff packs are gone for good, but we'll probably still get game and expansion packs.
Kits are probably the new stuff packs now because they don't need a dedicated team to make sure a gameplay aspect is working. They can just contract outsourced employees for each kit and pump them out cheap, fast, and easy. So it's a way for Sims 4 to still keep getting content, even with lowered staff and expansions happening less often.
I don't think kits themselves killed off stuff packs. I think Sims 5 is slowing down production for Sims 4, and kits is a response to that.