I don't remember arguing that developing a modern browser is cheap - I'm responding to the strange notion that the availability of a full open source browser from Google somehow makes things harder for competitors. It doesn't. And, again, as to forking - Apple created a competitive browser by forking an opensource project, Google created another one forking Apple's. This is demonstrably a viable and arguably more effective way of developing a browser.
Because they can outspend you in engineering resources to push forward, and working with someone elses stack is a burden, not a blessing, if the other side moves more quickly (and forces you to rebase).