They could reduce dependency by focusing on their core mission, cutting out the cruft, and using the extra money to build up an endowment that can last them forever.

Obviously, they didn’t take that option.

Their revenue is from Google because they chose to make a deal with Google. They could have made a deal with Microsoft or some other search engine provider.

That's one way to look at this. Another way is that Firefox lost market share, and is falling behind. Firefox is also the primary cash-cow for Mozilla. Given both, shouldn't Mozilla be investing in Firefox development to make sure Firefox doesn't fall further behind?

(Mozilla's non-Firefox products didn't actually make any impact thus far).