Like many things, the devil is in the details. United currently has multiple pricing schemes, though I've noticed in recent months they are harmonizing around a single interface/pricing model of 1hr/2hr/whole flight, and payable with miles or dollars. (The pricing per hour varies based on flight length, interestingly). Until that system is rolled out to all aircraft, it will be hard for them to sell monthly or yearly plans because some planes will recognize it and others won't.
I generally fly TCON flights, about 15x a year, so ~30 segments annually. Typically, I take advantage of United's "pause wifi" feature, and get email, download what I need, then hit pause. After that I can do all my work, then unpause, send emails, etc. Doing that, I only need an hour of wifi for a 6 hr flight, and the price for one hour is generally $3.99, or 30x$3.99 = $120yr/$10mo.
So, unless United's subscription is <$10/mo, I wouldn't bite. And I doubt it would be that cheap.
If you need constant connectivity in the air and/or you fly a lot of segments and have to pay over and over on every flight, I can see the calculus being quite different.