There's at least a couple of reasons that doesn't apply. Software projects don't develop linearly. 20 years ago the Linux kernel was 8 years old, today it's 28

There's at least a couple of reasons that doesn't apply. Software projects don't develop linearly. 20 years ago the Linux kernel was 8 years old, today it's 28 years old. The motivation to upgrade the relatively mature 2020 kernel will be much less than that to upgrade the immature 2000 kernel. And perhaps the biggest one - computers have changed. 20 years ago a PC that could run Linux was a big power hungry thing. Today it's the size of a box of matches. Embedded Linux PCs with limited upgrade paths are everywhere today. When I worked on satellites (stopped in 2015) we'd routinely use electronic test equipment which was running an embedded version of Windows 98. Given that the stuff shipping today is even further down the maturity curve, I think it's almost certain that it'll be used in 20 years (if it still works).