I’d be glad to do business with a company that cares. In my business, I walk away from prospective vendors that don’t demonstrative proactiveness or that just follow the pack.
I would absolutely expect them to be able to route around issues with a single bank in the same way I would expect Cloudflare to be able to route around issues facing a single data center. with that kind of business it doesn’t make sense to have a single point of failure with the money you manage. There should certainly be more than one bank in their portfolio, and the differences should be small enough to capitalize in an emergency.
If you’re not mitigating risks brought by your most important vendors, then you’re not mitigating your own risks. Any organization that’s serious about payroll will ask it’s payroll vendor about planning for various out-of-band failures.