Generally if you want long support windows you go for big boring brands’ simplest business class laptops. Or Apple.
Small companies an make a commitment to support, but they often have neither the money, customer base or manpower to follow through when the going gets tough.
I have found that popularity is a better predictor of spare part availability than any commitment from a company of any size. When they stop selling parts, there’s always the second hand market. When that dries up there’s always third party parts.
Firmware updates are one of the places that dell, Lenovo and Apple shine. Because of their customers expectations they tend to release new updates and drivers as functionality expectations or security conditions change.