This would be an absolutely insane antitrust nightmare and there's no way DOJ would allow it. DOJ didn't even let Visa acquire Plaid!
They're so similar* in large part because they're a duopoly primarily locked in competition with each other (at least, to the extent they're not collectively manipulating the market). If they merged, that competition would go away and pretty much everyone other than Visacard would likely suffer.
*The article you linked is discussing their similarity w/r/t the consumer experience specifically. Keep in mind that they're payment processors, so there are other relevant groups of users--including banks (who decide whether their cards will be Visa cards, Mastercard cards, AmEx cards, etc.) and merchants (who pay a fee to Visa or Mastercard every time a customer pays with one of those cards). I assume Visa and Mastercard are fairly similar for these groups as well, but I've never been a bank or a merchant, so I can't say for sure.
