The functionality is absolutely incremental--but it's the little differences, right, that prevent them from being used together cleanly without passing through

The functionality is absolutely incremental--but it's the little differences, right, that prevent them from being used together cleanly without passing through some other interface (say, a REST API boundary or TCP connection or something). To strain our analogy further--it's not like we've got a fundamentally new type of bolt every 5 years...it's that we've got slightly different bolt systems with different diameters that can't be mixed and matched on a project without making maintainers' lives hell. Oh, and in 5 years? The manufacturer is out of business--so sorry about your house door.