Not polluting the planet and being a rampant consumer for one is a very good reason not to upgrade.
If one is to actually use a computer not as a glorified web terminal, you'll find that for most things that aren't video/audio/image editing computing power really stopped being relevant after about 1998.
Image editing can be fast enough on a Pentium 3/4 up to 1280x800 and imagemagick.
For audio/video, a Pentium 3 or similar would be enough with a good video card, some mplayer/mpv settings to skip the loop filter on decoding and video should be good enough up to 720p on Pentium 4 with SSE2 or pre 2010 Celerons. Celerons from 2010 and beyond can play 108p at 30FPS without sweating.
EDIT: I use Slackware and OpenBSD too :D.
Fluxbox, Rox, XTerm, Mocp, Djview, MuPDF, Groff+MS, Bitlbee+sic... you don't need a high end machine, for sure.
For Rox:
mkdir ~/lib
cd ~/lib
git clone https://github.com/jun7/VideoThumbnail
mkdir -p ~/tmp
cd ~/tmp
git clone https://repo.or.cz/rox-lib.git
cd rox-lib
cp -r ROX-Lib2 ~/lib
Done. Add Zukitre for GTK2/3 themes, the Zukitre theme for Fluxbox (I have a fork for the theme with better settings),
lxappearance and maybe Elementary/Tango2+Tango icons as a base
for Tango2 icons, and you'll have an elegant and blazing fast desktop.
If any, you can install fbmenugen and config it to set some automatic XDG menu entries and some of them to suspend/hibernate your laptop.
I still have an old G4 powermac hanging around to do most of my image and audio editing in under (gasp) OS9 no less. I can still do huge modern-pixel-count images with Photoshop 6 just fine, because I don't really care about 10bit color or whatever the hell the kids are up to these days.
But then again, we're talking about computers, not media consumption machines.
If one is to actually use a computer not as a glorified web terminal, you'll find that for most things that aren't video/audio/image editing computing power really stopped being relevant after about 1998.