This is my thought, too. I'd bet customer perception is also a factor; there's a risk the old boards (not even made by Intel) die and cause problems, and Intel

This is my thought, too. I'd bet customer perception is also a factor; there's a risk the old boards (not even made by Intel) die and cause problems, and Intel wouldn't want to deal with press/comments like "This CPU stopped working after 2 months" or "I installed this new CPU, and within 2 months it killed my motherboard that had been working fine for 7 years". They've released CPU upgrades with the same socket before, I'm sure they have the sales data to know how that performs vs new socket. Laptops have outsold desktops for well over a decade, and their CPUs pretty much non-upgradable. I can't easily find a nice chart to reference, but intuition tells me the desktop industry is similarly trending towards complete "system" sales vs individual parts. In other words: Most people don't upgrade their CPU, they upgrade by replacing the entire system. If true, this also means the socket would be almost entirely irrelevant to sales performance.