In this case the sunk cost is time, the extra cost is money. So I think it kind of applies. They've just gone with a dual-currency system like predatory mobile

In this case the sunk cost is time, the extra cost is money. So I think it kind of applies. They've just gone with a dual-currency system like predatory mobile games ;) But I see your point, they're not necessarily overvaluing the time they've already spent, they're trying to avoid doing it all over again.