CPU gains for the last few gens have mainly been in perf/watt and, now, core count in lower-class chips thanks to AMD. The biggest benefit of a system upgrade for the last few gens, from a user perspective, has been for the chipset's gains - NVMe, USB3, USB-C, etc. more than raw performance. That's slowly trickled up but the real-world gains in many tasks haven't been enough to really justify it.
I know many people still sitting on anywhere from a 2xxx gen to a 5xxx gen and just don't feel compelled to upgrade from a CPU perspective. Those that eventually do, do so for the motherboard features more than the CPU - that's just a necessary cost for a small benefit.
This is marginally different for laptops where perf/watt becomes more important, of course. However for desktops, I certainly wouldn't be troubled by a 4xxx gen. I upgraded last year and it was from a 920 to a 6600K. Even the 920 did much of what I wanted, honestly, it was more a luxury upgrade.
I know many people still sitting on anywhere from a 2xxx gen to a 5xxx gen and just don't feel compelled to upgrade from a CPU perspective. Those that eventually do, do so for the motherboard features more than the CPU - that's just a necessary cost for a small benefit.
This is marginally different for laptops where perf/watt becomes more important, of course. However for desktops, I certainly wouldn't be troubled by a 4xxx gen. I upgraded last year and it was from a 920 to a 6600K. Even the 920 did much of what I wanted, honestly, it was more a luxury upgrade.