I don't think we're gravitating towards complexity. It's much simpler:

1. We add more functionality to the base of what exists (e.g. HTML). More and more individual features. (e.g. selecting things in the DOM, animations, effects, interactions, tracking state)

2. At some point someone takes all the patterns in the latest wave of functionality and writes a library for them. Library is widely lauded or adopted (see jquery, React).

3. We think of new features building on top what is now the new norm, it's now table stakes. Rinse and repeat.

This is a common pattern in engineering, for example writing new entries to a DB, then at some point going back and compacting.