> It seems that, if this was possible to implement (card tokenization obviously being an issue here), you could have multiple independent backend providers implementing the Stripe API as a provider, and you load balance across them
Doing your own PCI compliant credit card storage system instead of relying on your payment processor's tokenization system is not too difficult, but is a bit of a pain in the ass.
The biggest problem I see to switching between payment processors is the Visa and MC stored credential requirements. A few years ago they changed to a system where when you store a credit card you have to set a flag on the first transaction that tells Visa or MC that you are storing the card.
When you do subsequent transaction using the stored card, you have to tell them you are using a stored card and send them a reference to that original flagged transaction.
Most payment processors I've used give you a transaction ID that is something they generate, not the transaction ID that the Visa or MC network generates. If you are doing your own card storage that can lock you into using that same processor for future on-file and recurring charges on that card, because only that processor knows how to map the transaction ID you know (the one that processor generated) for the original transaction to the Visa or MC transaction ID.
Note that even if your current processor provides a way to get the underlying Visa or MC transaction ID there may be no way to use that with another processor. The other processor's APIs that take a transaction ID are going to expect one of their transaction IDs.
I'm not sure how widespread this is. Visa and MC announced their "Stored Credential Framework" several years ago. As the deadline approached for when it would become required, too many major payment processors were not ready and so it was delayed a year. By the end of that year there were still major ones not ready (Authorize.Net for example still hadn't even said what their plans were) and so it was delayed even more.
The processor we use was ready by the end of the first delay and we got our system converted to work with that, and I stopped following what was going on with the others so have no idea if it now actually is required everywhere or if there are major processors that still have an extension.