I don't think anyone has argued that building a browser from scratch is impossible, it's clearly not, just that building a competitive engine from scratch is impossible. SerenityOS is a sort of very cool art project, it's not attempting to justify itself in any specific way. If they make an engine that's 1% as good as Blink, works OK for the sites the authors personally care about and eventually they lose interest, OK, so what, no big deal.