As engines got more efficient, coal became cheaper to put to work, and so people found far more things to do with it. The same thing is happening with developer hours.
We've been running Rails apps in production continuously since 2007. If you treat software as anything other than completely disposable, it's been a no-brainer for the entire 19+ years I've been paying attention (not despite its age, but because of it).
The premise that you get meaningful efficiencies from JavaScript on the back-end just because you have to use it on the front-end has been pretty thoroughly debunked at this point. Instead you mostly get a larger blast radius when the front-end ecosystem has its monthly identity crisis. OP's "stacks-du-jour" and programming language "flavour of the month" framing is exactly right. A shocking amount of web software architecture is just following fashion trends dressed up as technical decision-making.
Most of the churn in tech stack isn't driven by engineering requirements, it's driven by résumé optimization and Hacker News anxiety. Rails has quietly been powering serious businesses the whole time. Does anyone think NPM's 3.1 million packages enable more functionality than RubyGems' 190,000 packages?
This looks interesting, but I'm stuck on step 4 of the web setup: where do I get agents to start with? Shouldn't there be a default one that can help me get other ones?
FWIW I've had success with self-hosted [LinkDing](https://github.com/sissbruecker/linkding) and the firefox SingleFile plugin (so it archives what I'm seeing / gets around logins etc). LinkDing also links directly to Internet Archive for any URL.