One can say that 'incredibly proprietary' was wrong, it was less proprietary then many other CPU at the time. It doesn't really change the analysis. Practically speaking that gave most Sun costumers very few option, and increasingly less as the 90s went on. There were just to many RISC ISAs competing for to little market. Sun tried to play the vertical integration walled garden game and in that game its really, really damn hard to win.
Even if v9 isn't an IEEE standard, you can still pay Sparc Intn'l for a license. Try going to intel with a request to make x86 chips.