Here's the thing: WotC isn't going to produce a Dungeons of Drakkenheim (just as an example). Try to go the GSL-like route reduces the probability anyone else will.
I share that concern, but let's probe it a bit. That particular Kickstarter campaign brought in about $1.25 million with 13,000+ backers. Kickstarter takes 5% (someone correct me if that's wrong).
@Morrus suggests we could double that to estimate total sales, and let's assume those additional sales are all print products through traditional distribution channels at a 60% discount, just to make this scenario as unfavorable for the publisher as we can.
So total (net) sales are 1.25*.95 + 1.25*.4 = $1,687,500, and we'll just assume that's all in one year. That's $937,500 above the royalty threshold. Let's further assume that the royalty isn't even graduated--it's a flat 10% of net sales above the threshold, so that's a debit of $93,750 on the campaign. If you're the publisher, would you still do the campaign?
If we assume Ghostfire wants a 60% gross margin (that's what FFG wanted back in the dark times pre-Kickstarter), that means their budget to produce the book and add-ons for the campaign is about $675,000. Can you find room in that budget for a $93,750 royalty? I know production standards and associated costs are much higher than they were when I was in the game, but it seems like you definitely
can? You'd rather keep the money, obviously, but it doesn't seem like that transforms a viable campaign into a non-viable one.
I don't think Wizards is going to go with a flat 10% royalty on net sales above $750,000. But even if they did, I think there'd be reason to expect that campaigns such as
Dungeons of Drakkenheim would still happen.
ETA: corrected a math error.