I suspect the trick here is that capitalone issues so many other cards that once they standardize on it, merchants can't refuse their network without losing substantial amounts of business.
From a competitive landscape, this is probably a good thing - Discover never really did much issuance of cards by other banks, and this means competition against Amex.
Better marketing around this would likely improve signups.
I've had one in my wallet since college; it has never been a primary card for me. But I recently needed to choose a new one and scrolled through the options on my phone. From the small photos, I chose something that looked like some sort of abstract painting. When it came in the mail, I realized that it was not a painting at all. It was way more interesting than I had imagined. And indeed, it is the only credit card I've ever had that has received compliments. Does that influence how often I pull it out of my wallet as opposed to the other cards in there? Perhaps...
For 2023, the Capital One was the fourth largest issuer[1] by purchase volume. Post merger would put them at number three. The challenge isn't signups, it's retention. Discover being in the subprime category means it's often a borrower's oldest account and thus likely to be sticky. Capital One is no slouch either.
But also: this may not even be a credit card play. Matt Stoller points out that there's a loophole in the Durbin rules for debit cards that hasn't been a factor until today: the rules that cut debit fees in half in 2011[3] only regulate networks operating as middlemen between banks and merchants. So presumably they can take those fees and double it again, at which point it's a fascinating question of who blinks first: merchants or Capital One.
[1]: https://money.usnews.com/credit-cards/articles/biggest-us-cr...
[2]: https://www.thebignewsletter.com/p/the-fed-is-behind-the-cap...
[3]: https://www.federalreserve.gov/paymentsystems/regii-average-...