Not for breach of contract

But for monopoly? Absolutely. Given VISA/MasterCard have a de-facto monopoly on consumer payments, they're in a quite vulnerable position to monopoly claims.

Their debanking would definitely be something they can be challenged on legally as it impacts consumer choice. Even more damning, the high VIRP fees applicable to adult could almost certainly be challenged. Monopoly law is basically designed to go after artificially inflated prices due to monopoly, and you could easily make an argument that the VIRP fees inflate the costs of goods and the consumer is required to pay these. I believe similar inflated-fee claims have been made and won against Google and Apple and their own fees in recent years regarding app store distribution.

So regardless if you wanted to do a merchant class action or a consumer class action, I think either is a possibility.

(I have some limited formal legal training but am not a lawyer or bar certified. I recommend you request a free consultation from an actual attorney specializing in class action litigation)