The Signal team has always been open about the reason why they reject third-party clients: they claim that XMPP adoption was hindered by the inability of a user’s software to know if the software on the other end supports the same feature set. XMPP had grown into a large set of features that some clients supported and others did not.
If Signal introduces a new feature, it knows that all users’ devices will support that feature, because its own software is the only game in town.
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.
If Signal introduces a new feature, it knows that all users’ devices will support that feature, because its own software is the only game in town.