Having read the terms of the contract, and having spoken to a decent segment of contracted writers over the past week, you're lucky to make anything at all after the initial guaranteed payout period. The ones who make 5-6 digit incomes from doing this could probably be counted on two hands, and of those, they probably only actually saw about 10% of what their story actually earned, if that. And that's before taxes.
I mean, if they're happy with that, great. I want people to be happy, and 50-70k isn't too bad for a 10% royalty. That's decent money for a legit contract advance irl. But that payout rate on physical books actually takes into account physical irl marketing, physical irl printing, graphic designers, print layout, focus grouping, and a bunch of other expenses WN doesn't have at all or has a significantly reduced cost of. When you factor that in, plus the obsene amount of writing that you have to do to not lose the contract, that kind of money is actually very stingy, by standard publishing levels.
That's just my two cents, for whatever it's worth. If it makes you happy to be contracted on here, and it makes you money, and you're not able to land a "real" publishing contract yet, then good for you! Some small success, even under very unideal terms, is better than no success!