LuaJIT has ~85k lines of code. Chromium and Firefox have somewhere in the neighborhood of ~30 million. If you need ~1 developer for "a LuaJIT" then you need ~350 developers for a browser.

The assumption that a browser needs ~30 million lines of code may be false.

It may be possible for a scripting language with at least the same features as LuaJIT to be as fast as LuaJIT in less than 85K lines of code.