What could we possibly do to limit the damage from this? Malicious Chrome extensions can also rewrite entire web pages, so if bad actors purchase or infiltrate legitimate extensions, they can also put users at risk.

We are considering continuing work on our own browser, since we could ship certificates to use client-side, but it's a recursive problem since our users would have to get the first trusted version before they could trust updates to it. If they have trouble reaching our site in the first place, through poisoned Google search results, poisoned DNS, and poisoned IP routing, then it is difficult to do this.