I'm having a job thinking of any of the "surprising number of viable business models which involve undercutting the incumbents for payment services." Outside of fintech I can think of companies using LVLT's fiber network and not paying much. Also travel companies booking off peak hotel rooms and paying enough to cover maid service etc but not the cost of building the hotel. Both of those happen where there is an excess of capacity in a competitive industry. When things turn and there is a shortage, hotels and the like can jack their prices.
In the mobile industry all the service providers and virtual operators are such examples. They get to use the physical networks of the incumbents for a governemnt regulated price in most markets, to create competition an make sure the incumbents are not exploiting the natural oligopoly in the mobile industry (due to the cost and feasibility of several players with nationwide antenna-coverage).