I think there's a point of diminishing returns. I've lived through the growth of the computer industry and I can say a computer from 2001 can still meet the needs of many users. It's software which has obsoleted those old computers, not the users needs.
Well, if by software you include browsers... Cuz it's basically impossible to browse on most 2005 computers, much less a 2001. In 2001, I was using a G3 iBook. In 2005 I was using a G5 iMac.
Even with a modern browser that can run on the 2006-2007 Intel computers, those machines are often very problematic because of the performance issues, partly due to the fact that a lot of these computers don't have hardware h.264 acceleration. You often can't even watch a news video without bringing your computer to its knees.
You can run MS Office 2011 though I guess.
BTW, even in 2001, the G4 iBook was kinda slow. The G5 iMac was OK in 2005 though.
My 2007 8-core Mac Pro is OK performance-wise in 2021 though.
We still have some Pentium 4s with XP at the office to run old, expensive equipment. Gotta say, those are pretty painful to use.
2009 with 45nm Core 2 or 1st gen Core i-series ain't too bad though.
We still use a 2008 MacBook Core 2 Duo 2.0 and 2009 MacBook Pro Core 2 Duo 2.26 with SSD and 8 GB RAM as well as GPUs that support hardware h.264 acceleration, but while usable, honestly they still feel quite slow even just browsing the net to look for recipes.
OTOH, I finally put an SSD in my 2010 Core i7-870 iMac with 12 GB RAM yesterday, and it basically feels almost like a modern machine. The main thing holding it back is the fact it is stuck on 10.13 High Sierra, but even then it's fine.