When you're critical of something, you may want to avoid blanket statements like "it's not worth it.", I could see a cheap 6 core 12 thread CPU being worth it to alot of people, myself included… people who do a bit more than gaming, or people who realize that gaming CPU demands are starting to change in favor of high thread count CPUs.
My opinion..I think anything over 50-60% is a good jump in GPU. For example its still worthy to buy GTX 1060 or RX 480 over my gtx 960 which is around 60% better
For cpu however i consider it should be 100% better as i mostly game i dont need very demanding cpu.
Choice for most people usually depend whether they are satisfied with current hardware if yes no need to upgrade and also mostly their budget.