as I understand it, Discover gets to charge higher fees on debit cards (than eg visa and mc) due to some regulatory carve out, so capital one wants to take advantage of this and Discover’s payment network
However, Discover is (largely; I believe at least Pulse is available to issuing banks other than Discover) also a three-party network, which is inherently exempt from the Durbin amendment: https://www.congress.gov/crs-product/R41913
The effect of this is that merchants end up hating taking Amex since it costs more, and I foresee Discover (which is even more niche) ending up the same way - there will simply be no reason to bother accepting Discover at all. How many people only have a Discover debit card in their wallet and no other?
I shop for name brand stuff at Winco, a grocery chain in the Northwest US that doesn't take credit cards and makes you bag your own groceries. I'll be curious to see if they stop taking Discover debit cards if the fees increase.