This was obviously coming for some time. All of the new Solaris development since Oracle took control has been very meager. As an example, there have been no system ABI changes in the past six+ years (which is why OpenIndiana can run Solaris 11 binaries, and vice versa).
Especially given that Fujitsu is closing down their development of new UltraSparc/Sparc64 processors (they were by far the most active UltraSparc OEMs), and since one of Solaris's best features was great UltraSparc support (better than any other OS, way better than Linux), it makes sense that Solaris development would slow or even stop now.