Except the fact that nobody (relative to even their current userbase) would use it, and maintaining a browser is incredibly difficult and expensive.
It would be the death blow to their market share, which would destroy Gecko as a viable browser engine (not enough users to get websites to care about the bugs, or even necessarily get the bugs reported).
The only way that would work out is if they gave up on Gecko and switched to WebKit or Blink.
Their choice of business model isn't really much of a choice, it's the only viable option that gives them any influence whatsoever.
But the reality is that because of this, browsers are commoditized, and the average user will never pay for a browser if they can get Chrome or Safari for free. That's probably true of the average HN user, too, for that matter.
Assuming 100 people needed for Gecko, and $150k/year annual, world-wide, average developer expense, we come to $15M/year. Mozilla already has about ~$50M/year non-Google revenue from its products (coming from "true" users/customers).
Does Gecko really need more than 100 people?
This is a business model question, right? Nothing prevents someone from making a great privacy focused browser and actually charging for it vs being directly (Brave?) or indirectly (Chrome, Firefox?) ad-monetized.
Also in this context, referring to "customers of privacy focus products" is technically incorrect, they are actually users. Definition of a customer is "someone who pays for goods or services" thus Mozilla's main customer is Google (accounting for close to 90% of its revenue). Maybe looking through this lens, relation of Firefox product direction and what its "customers" want becomes more clear.
edit: simplified for clarity