Apple and Google wallet are integrated into the credit card processing infrastructure. So they don't compete with Visa, their payments still go over Visa's rails.
Plaid's killer feature has always been their private access to APIs that the banks won't give to anyone else. It's essentially like they're using modern web APIs while everyone else is using snail mail (i.e. ACH, bank transfers that settle in batches once per day). I'm not exactly sure how they struck those deals and why there is no reasonable, open integration path or payments protocols between banks in the US like there is in every other developed country. Is it just that the banks like making their $15 per wire transfer? It feels like there's got to be more to it but I'm not sure.
But regardless, where we've ended up is the banks have fought so well to keep everybody out of their collective walled garden, but somehow Plaid has snuck in under the radar and has become so large and entrenched that they are themselves being called the monopolist, rather than the banks that own and control the system but of which there are multiple. I don't know enough about the details of Plaid's business though to weigh in on whether there's any truth to these claims.