It doesn't matter that they can, and want, to invest as much as they do today in the browser space. Microsoft's IE was also the most competitive until it killed off the competition, then they decreased quality.
Back then the decrease in quality was by way of not developing new features, or developing them in a quirky way, that was all well form them because they didn't want web applications to displace native Windows applications.
The decrease in quality you may see from Google may come in privacy, they'll be able to tighten the surveillance as much as they want once no website you care about works outside of Chrome and you can't switch.
Yes and it was incredibly difficult to displace the monopoly of a vendor that had incentives to make their product, ultimately, unattractive in every regard.
Imagine now the network effects and a browser that actually keeps up to date, they'll just have won, period.
