How is that "threat" a problem? That sounds like an Old Microsoft objection to cross-platform code, but the web is already cross-platform code, so that doesn't make any sense.

And if the browser vendors really didn't like it for unknown reasons then they could have just stopped supporting Java in the browser, as has largely already happened for various other reasons.

This is one of those "we all must hang together or we shall all hang separately" situations, and they apparently decided they'd prefer to hang separately.