SA is only successful in that it makes those realms that adopt it popular. It doesn't get them results. In practicable terms, the only realms currently capable of achieving anything are Asylon, Aurvandil, Barca, Luria Nova and the Falkirkian Freestate. All other realms are gridlocked, and manned by small numbers of complacent nobility, who are often unable even to maintain their land.
Forming gridlocks and being popular is good for nothing. It's detrimental, even. It poses, to the characters, an existential crisis -- what are they there for? What are they meant to achieve, in an environment where they cannot lose. An environment where they cannot win. Many come to realise that the answer is nothing, and so they leave. One year from now Iashalur will have lost several of it's regions, despite having zero external threats. Same goes for Corsanctum and the Libero Empire. The nobles will just trickle out, at the same steady pace that they always have, and those who remain will be left weaker. But even that is meaningless, because, with or without the decline, the environment for that remainder is exactly the same. There is no one who is even going to bother preying on them, that is how futile their existence is. Fat lot of good popularity did them.
Morek Empire and Astrum are just a step above that. They are lucky enough to be on the fringes of the popularity zone, so that they can keep up the pretence of having an objective by engaging those outside of the bubble. But that is only a pretence. Sending 5 characters, each, to wage a war on the other side of the map is the entirety of their purpose. That may be enough 'excitement' to barter an existence of minimum sustainability from it's characters, but that is all. They will remain popular realms, without practicable value.
And so it is to Asylon, Aurvandil, Barca, etc. that characters will flock. It is there that they will stay. And it is those places which will maintain a raison d'etre. One year from now those places will be as exciting as ever, while the popular realms will be, well, popular.