I have been using gkrellm for at least the last 25 years or so. I very much like being able to see what my system is doing at a glance and no other system monitor comes close to being both as detailed and compact.
Same here. One of my favorite plugins is BubbleFishyMon which seems funny (and it is: water, air bubbles, fishes and a duck indicating system/memory/network load), until one realizes how much information it can convey with just a brief glance.
At least once a year for the past couple of decades I'll receive an email from a user that makes me want to modernize bfm¹. However, I've never managed to get an auto-scaling version to work with reasonable performance, which is unbelievably annoying for such a simple tool. You can sort of hit the target for gkrellm-bfm while still depending on GTK2, or for the standalone bubblefishymon wmapplet where you can use a modern GTK² or jump straight to SDL.
Never in a way that doesn't noticeably chew additional CPU though, and definitely not for both use cases. It upsets me each time I reply "no, not happening without patches". Perhaps it is time to release an Electron version or one that just burns a whole CPU core so it isn't lost forever ;)
¹ It used to just be people with crazy setups, now it is basically any screen given their higher DPI.
² As it is right now I don't even have a system that I can use last release on.