Honestly, I have built around 6 PC so far now, over several years. I never once decided to upgrade just the CPU without the motherboard. It never crossed my mind to keep a mobo for more than 1 CPU generation.
There was always something I wanted from newer mobo generations, newer USB 3.0, new SSD SATA support, new SSD M.2, new 2.5Gbit ethernet, new USB-C, etc.
I'm sure some people just upgrade the CPU while keeping their mobo, I just never found it to be really worth doing. I believe most people who "upgrade" upgrade their system, including mobo.
I think the "dead socket" is kind of an overblown argument, as I believe most people upgrade their mobo too when they jump on a new CPU.