That Visa isn't fighting this should validate that the government's antitrust enforcement has been lax. For a merger valued in billions of dollars, hiring even the best lawyers for a long fight would have been a rounding error. The only way this happens is for Visa's lawyers to think that the government would likely win.
With public market valuations of B2B companies today, I could see Plaid being worth considerably more than the $5 billion that Visa agreed to. I think it's less likely buyer's remorse than just overwhelming evidence of anticompetitive behavior. The original DOJ complaint [1] has a lot of direct quotes from top level Visa execs. See paragraphs 9 and 10.
No, they always knew the 500x valuation was BS. It's pretty much like they said, it was a defensive acquisition to prevent the data from going to any of their closest competitors. Visa had no idea what it was going to do with the data, but just wanted to keep it out of everyone else's reach.
