I thought the 3 year thing was standard for many years, pretty much as long as there have been retail-boxed CPUs.
I think the logistics of a long term CPU warranty might be difficult too; aside from a few perennial, typically embedded market, products, manufacturers want to close out product lines and keeping warranty spares for decades becomes a burden.
Imagine the fracas at the RMA office: "He says he doesn't WANT a new 14900KS, he wants a 50MHz 486SX2 to replace the one that blew out, and he has the state attorney general on line 2."
Also, the overall cost of design and production is going up for years (>10 years already or so), which is why only these few fabs are left that run these advanced processes and they need massive scale to stay profitable. This "going up" is btw. non-linear, more like exponential and that also applies to power consumption of the resulting chips if you want to eek out "a bit more".