It isn't clear if graceful degradation is possible in a security app.

There have been tons of problems with that concept and TLS

Cases where a client is completely broken are never the problem: users will be forced to switch to a different client. It sucks, but it's no worse than the current state of affairs. A security-mandated change in protocol/behavior would fairly fall under that category.

You can always define backwards compatibility that only goes to a certain lowest common denominator feature set, and no lower. For instance I have a number of httpd that support TLS1.2+ and specifically disallow SSLv3, TLS1.0 and TLS1.1. The population of browser user agents that don't understand TLS1.2 is infinitesimal at this point.