Open browsers have been around for decades, Chromium didn't offer anything new there and as you say it even started off an open browser itself. If everyone did what you suggest there is a net loss for the user as where there used to be competition in browser backends there is now whatever Google considers in it's interest to merge into Chromium for others to also adopt (until the point someone manages a fork and we're back at the part where you say companies should drop their engines for what is popular instead).