I think there's a big machine between a 2016 laptop in 2026 and a 2006 laptop in 2016.
The 2006 laptop is absolutely useful today. Truth be told, there's nothing it can do that an iPad can't do; but nonetheless, it's far from useless. At least if it was a premium laptop like the MacBook or MacBook Pro. Now if you go one year prior you're running into PowerPC which is a little more obsolete; but not at all useless. It's just harder to find software because nobody has been developing on that platform for many years.
You're running an older OS, you have way less performance, etc, but you can still do a lot.
1975 to 1985? Might as well be 100 years. In 1985, a computer made in 1975 is useless. In 1995? That 1985 commodore is absolute garbage. But things have slowed down since then. And I don't think that trend is going to stop. The push is towards mobility is big. Heck, in the last three years, there haven't been significant advances in CPU technology. They've just gotten cooler and more efficient, but not really faster. A 2016 MacBook Pro in 2026? It'll probably not run the latest version of MacOS and there may be some breakthroughs that it'll miss out on (for example, that 2006 MacBook is going to lack a 64-bit CPU, thus be permanently limited to 4GB of RAM no matter what, be limited on what software it can run, and generally perform much slower than the 64-bit CPU's). But for basic productivity, it'll almost certainly still be useful.
Honestly, sometimes I fire up my really old stuff just for fun. There are people on this forum (scroll down!) still using OLD PowerPC stuff. Sub-500MHz stuff. Still useful for certain tasks. Everything I've done this morning (replied to threads on this forum, answered e-mails, etc.), save for the HD YouTube video I watched, could still be done on that G3 Power Macintosh I had in the 90's running OS9. Wouldn't be done very fast! But it could be done. I've got the 2016 MBP 15" on order though for Photoshop, Final Cut Pro X, and similar tasks. Which certainly can not be effectively accomplished on a 10 year old laptop. Thus, "useful" is really relative.