It's a metaphorical eternity, not a literal eternity. Four years ago is 2019; I use plenty of software from then.

And if I understand the license correctly, you're perfectly free to use the software in your own open-source projects, and your commercial projects unless they're a re-packaging of Hashicorp's own, etc. We shouldn't act like this is an evil business decision, or like this is morally equivalent to proprietary software that does stay proprietary forever with no apology or caveats.

Four years u patched with pote trial security issues? Probably depending on four year old libraries which are annoying to get, based on build.tooling which is outdated ...

As a starting point it’s a great line; rather than closed source have something that worst case you can use in four years.

As a restriction added to something previously open, it’s horrendous. AGPL would be better.