The core will have to still be a solid battle tested thing. Imagine having to worry about if your vibed browser is quietly dropping some elements. Firefox and Chromium browsers are valuable because they are the targets, if a site doesn't render properly in them, it can be said that the site is wrong.

If everyone runs a vibe coded browser engine that evaporates and you might as well just have sites serve a bunch of markdown for LLMs to render in whatever inconsistent way they choose.