The reality I keep running into: software that "just works for years" requires dependency hygiene at the ecosystem level, not just the application level. You can write Common Lisp or C or even most of Go that way and your code will still run in 20 years. The moment you depend on a modern frontend framework or even a modern backend one, you've committed to following its release cadence — which is often "we deprecate things twice a year."

Framework authors have their own incentives (relevance, employment, hiring funnel) and aren't optimizing for your project's longevity. The only way to write 20-year code today is either (a) work in an ecosystem that genuinely values stability (Lisp, C, parts of Erlang/OTP, Postgres) or (b) accept the tax of a modern stack and budget for it explicitly.

Most teams do neither, which is when projects rot fastest.