The diversification of the pool makes me think that in the coming years we will see some FinTech come in and bridge between the various parts and cut the ~3% fee down to a lower overall amount to try to gain a larger market share. Especially in businesses with narrow margins, reducing that fee can make a huge difference, so I think the market would be there even if some drawbacks. Not sure how technically/legally difficult it would be though?
If there's already very little margin left in interchange for fintechs, what's to stop further compression in pursuit of customer acquisition for higher margin products? End state... is interchange going to zero?