Alternative browsers raise money in proportion to how many people use them. Google funds Chromium development and advertises to get people to use Chrome, so Firefox market share is eroded until they don't have enough to sustain themselves. Meanwhile Google pushes through changes to web standards at a rapid pace which consumes smaller competitors' resources implementing them. Also, Mozilla in particular is a mismanaged organization.
If you take Google out of it, Chromium gets to be its own thing. Then Chromium and Firefox compete with each other on equal footing and people use whichever one is better, but they're both actually open source instead of "Chromium is open source but everybody actually uses Chrome" and the latter isn't controlled by the company trying to break ad blocking and push DRM.
In the narrow case of zero day vulnerabilities perhaps, but a good ad blocker is part of any reasonable security posture and is key to avoiding exploitation in practice.