I'd definitely be interested in the service. I fly UA a lot for transcons SFO-IAD to visit family, and always purchase WiFi as it saves me a vacation day sitting on a plane. I typically do full flight, as my mode of work requires access to an online messaging app.
Of course, it'll depend on pricing and the number of flights needed to break even.
One other minor note, typically subscriptions count toward PQD, while wifi purchases do not as far as I know, so a possible small advantage.