I feel like with the advent of vibe coding, the bar to making your own browser is gradually lowering. People are already making all sorts of useful "houseplant programming" to serve their own pain points. Sooner or later, agentic coding will actually become really good and people will be able to just vibe code a whole browser in Rust or something that addresses the exact pain points that power users have.
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.