And compatibility doesn't just go back one release, or just prior Solaris releases. It also covers binaries from SunOS 4 (on sparc, that is). As an example, we have an old copy of microemacs
-rwxr-xr-x 1 root 139264 Apr 17 1989 emacs*
(although it might be even older).
That's a 15 year old binary in regular daily use.
During the Solaris 8 beta program, we discovered that it no longer worked properly. So Sun fixed Solaris for us so that it did. No ifs. No buts. No excuses. Solaris got fixed.
And that it broke at all is unusual. In a decade of running Solaris on sparc, and 5 years on x86, our experience is that applications just keep working. We don't need to requalify applications on the latest release, and can thus roll out new releases of Solaris with confidence.
So why is this a double edged sword? Well, I've been trying to wean users off using this binary for years, and it just keeps working so they aren't forced to switch...