When Maxis first started making kits they stated on Twitter that kits weren’t going to affect pack releases. They also said kits were designed to fill content gaps. I’m not sure if fewer packs last year (and possibly this year but it’s too soon to say) were the result of the pandemic, the Sims 5 development or something else (or a combination of things) but I think kits are supposed to be the solution rather than the cause.
I also think it’s too early to declare stuff packs dead, there was a 17 month gap between My First Pet Stuff and Moschino Stuff and they weren’t dead then (also fwiw the gurus have said they aren’t dead).