My previous company (ITA Software) made enough revenue to (rather comfortably) justify the ~$700M valuation Google paid for it. ITA never took in a single cent of advertising money or money derived from selling users' data to third parties.
I believe a vastly better email platform has inherent value, just like a vastly better travel planning system has inherent value. Look at the dozens of ways companies are currently earning their keep in the email sector. It's not the same as Twitter or Facebook, which only have usability value to consumers, and which can only succeed at massive scale.