This is the actual problem at the heart of it all. And even if it were more profitable to take subscription fees than to serve ads, what's stopping you from "double dipping" and serving ads anyway?

Very likely. I'd pay hundreds of dollars a year to Gogle if they guaranteed* me, with severe legal repercussions otherwise, that they wouldn't track me, or allow a single bit of my data, anonymized or not, leave their servers, or be used in any other way that wasn't for my own purpose.

Re-selling digital personas as commodities must be far more lucrative.

I actually wonder about this. SO's typical user is tech-savvy, and I would imagine many access the site with adblockers on (I do). So I suspect my value to the site in terms of ads is close to zero. I would happily pay a monthly subscription to know that the service will remain, given how much value I derive from it, if that gave me the assurance that they won't track me with ads/cookies/fingerprinting.

Their other income is from job ads, and I guess the value is that they have lots of data points about their logged in users (with scores high enough to imply they've interacted with the site a fair bit), in the form of what is posted, worth more than the aggregated list of websites that a user sees (as reported by ads).

I'd love to know more about this, as I have very little understanding of the economics of serving targeted ads. How much can they be making from ads?

But they're mostly wouldn't pay in ads either. The difference may be pay-for-ad-free vs adblock-and-no-money rather than getting more ad views.