As for SA, I don't buy it. "Blanks for people to fill" are, in my eyes, pretty much "blanks forever so as to not ruffle any feathers". May as well praise Eretzism for "leaving blanks for people to fill". SA still doesn't have a vision on the afterlife, as far as I know, which would be the number one concern of any religious person.
The debates are still happening on this one. There are advocates of a couple different theories, and none of them have managed to take the lead and force their viewpoint on the religion as a whole. But that's a good thing, I think. If they ever manage to do that, then that will put an end to the debate.
As for Eretzism, I really don't know much of anything about it. But if you are going to compare the two, then I'd have to ask: Do the followers of Eretzism debate theology? Do they argue about the afterlife? Do they argue about the nature of the gods? Do they debate on how many gods there are, and whether or not any specific god exists? I would suspect that the answer to all those questions is "No", and that Eretzism simply doesn't talk much about anything at all, other than how they can help advance the nationalist agenda of Enweil. Yet these are all debates that have occurred within Sanguis Astroism within the past few months.
So in a way you are correct. The very fact that blanks exist does not guarantee that you will have a successful religion. They are an enabler. It's what the players do with them that counts. In Sanguis Astroism, the players often use these blanks as enablers around which to hold theological discussions.
All it has is vague theology that doesn't really force anyone to do anything they wouldn't like to do. All can basically "worship" the stars in whatever way they wish. In my eyes, that's a hollow religion. Follows the same mold as all other "successful" national religions on other continents, except that it came first on the continent, and in a theocracy that would eventually colonize a lot, and at a time where the metagame was really pushing for religion to be taken seriously. That doesn't make it a good model, in my eyes.
I won't deny that the "came first" part helped. But by itself that simply can't be the only factor, or even the main factor. Other religions were founded in the religious vacuums of the west, the south, the central islands, etc. These religions had a wide open space in which to expand. There were no competing religions to stop them. The peasantry were all pagans, with no competing priests to steal their followers, no other realm-based religions to fight against, the nobles of the realm did not already belong to other faiths, etc. They had the
exact same opportunities to establish themselves and expand that SA had. Why did they fail? And I refuse to believe the answer is "SA was established first". That's a lazy answer, laced with sour grapes.
As for fewer old religions... A lot of them died on BT. I don't see any improvement.
And what have the remaining religions done to capitalize on this new power vacuum?
