Obviously, it doesn't need adoption to "survive", but it does need adoption to succeed in its goals: rescuing users from stupid hack-job login systems.

I would compare it more to Chrome: Google made Chrome with the goal of getting the web to work better for some people, so they could more easily consume Google's various services. You could say that Chrome's bottom line was "increased Google service adoption": if Chrome renders faster and provides more HTML5 features, then Google gets more and happier customers for their products.

Mozilla doesn't have "services" that Persona is a loss-leader for, but there's still that same ROI calculation of whether Persona is achieving its goals, and measuring adoption rate is a good proxy for that.

The important thing, that Mozilla failed to do, was to actually understand what an adoption curve for something like Persona should look like, and what things affect it. They needed to heavily advertise it (to developers, not to users!) over about two years, and then start looking at the adoption curve.