What exist is code that can be maintained, and code that just cannot (in practical time). There is no sharp cut, is a continuum. But if nobody cares, at some point it grinds to a halt.

Software has been around long enough that many companies have been burned by bad code and so they are willing to give training and time to making it better. While there are trade offs, spending several years of no features while engineering rewrites the whole thing isn't acceptable.

Extensibility is the term that speaks to this situation. Extensible code, updated by skilled developers, will allow for radical business logic changes without sacrificing the quality of the code over time.

It's like a skyscraper. The critical structural stuff is fixed but the architects and engineers try to minimize those pieces and create flexible spaces that can either be high density cubicles, enclosed offices, a cafe, etc...

Eventually it's true that you have to tear it all down and there's nothing wrong with that - a good developer will know when that makes sense based on the needs of the stakeholders and make that recommendation.

I like it, but I think how easy it is to replace is more important than how easy it is to extend. Otherwise it might end like those massive class hierarchies instead of a function.

For me, pure functions and enforced types do the trick.