I don't think that's true, old HW doesn't always degrade, most electronics works or doesn't work. If you clean the fans and replace the heatsink compound then, especially if you are dealing with a well made item, it should work as well as ever. Also you can swap in new parts, but it won't suddenly start running better in general. Not like an engine with moving parts that slowly wear (or donkeys etc that get old and don't go as fast

). It's possible to pick up a few non-terminal issues, like capacitors changing values over time, but generally things are either good, open circuit or short circuit. (Or intermittent which is mostly still not going to degrade performance, just annoy you.) In fact the likelihood of failure in electronics is higher in the first year or two of use than later, when the weaker units have already gone.
A lot of the performance problem is lack of support in the entire OS and web code for older hardware. As support for new computers is added, the old ones get slowly forgotten. It's mostly a software thing, but not just the browser, things like the compiler, kernel, graphics stack, desktop and so on.
Earlier this year using a P3 I found that Firefox ESR 45 (I think the last version to support P3) worked better on more sites than the last available Pale Moon SSE build, which was newer. I don't think this was the case when it was current because I seem to remember SSE Pale Moon's working well at the time of release. So I suspect websites in general may have some residual support for the last non SSE2 Firefox built in to them. I think this project could be interesting to try though.