Having other sources of revenue outside of a benefactor whose interesting in you relies on having a relevance in the browser market with them also actively engaged in implicitly or explicitly undermining that with a product of their own doesn't seem like too bad of an idea.

It's dangerous to depend on a single company (that also happens to have a competing product).

What happens if the money stops coming?

Developing more than one revenue stream is likely to be a more resilient strategy than hoping that Google is happy to keep paying a competitor browser to stay in business

Maybe, but that does not mean that they should put all their eggs into one basket.

Not when you're up against billion-dollar budgets.