I actually think it's higher than some people might guess, albeit with many travelers on corporate discount fares. (Large-corp discounts for premium cabin can be high...30-40% off the rack rate shown on an Internet search.)

I think there's a lot of paid J travel out there, and then some subset of those people use upgrade instruments to get to three-cabin F. There's perhaps *less* paid F travel, but if J is the moneymaker then the airline is still doing quite fine when people upgrade J->F.

At a couple of my large-firm jobs, we'd occasionally get a promo email about confirmed upgrades to F when we bought J. Sadly I never managed to snag the routes/dates when it applied. My paid J travel was South America or secondary Europe markets where there were only two cabins to begin with.

On some routes, airlines also seek to sell J seats to leisure travelers, something they didn't seem to care about doing the past. Pricing in the $2500-3000 R/T range for Europe, for example. (With enough restrictions to segment them from business travelers, of course.) In the past, more of those seats would stick around for upgraders...now they're getting sold.