I am not talking about the gateways. I am talking about the identity providers. If your identity provider goes out of business, revokes your access, or any number of other things, you lose your credentials to all sites.
For instance, here's a site with a guy trying to sell you on using IDPs from a big company rather than a small. One of his points though is that the IdP is a single point of failure, exactly my point:
https://www.tbray.org/ongoing/When/201x/2013/08/14/FC2-Singl...
Under the heading "Other Failures."