Developers wanted to build and deploy apps to end user machines. The round trip for page loads was lousy for usability.

Java applets were too shitty. Flash was too janky and hard to work with. So Mozilla started adding JavaScript as a hack. It filled a need.

a barrier-to-entry that makes it difficult to develop new browsers,

It definitely adds a barrier to entry, but JavaScript was really perfected in chromium, which is a different codebase from the folks who proposed and built js to begin with.

I’m not saying JavaScript is good, but it fills a need.