The Architect doesn't talk about stopping the blight (the disease, lower case), he talks of ending Blights (the scourge of darkspawn amassing around an Archdemon, upper case). His reasoning is that the Blights are just as devastating for his own kind as they are for humanity.
In The Calling, he says that his ultimate plan is to have every living being infected with the blight, and then those hybrid creatures would use their Call to find the Old Gods and destroy them before the actual darkspawn can wake them. BUT by DAA his plans have apparently changed, since he doesn't mention that, or at least there isn't enough space or context for a full explanation. As I recall, it's pretty brief in the game itself. He instead says that he wants to use Grey Warden blood to awaken his darkspawn brethren, which is what he's been slowly doing all this time.
I'm not sure which plan is more viable, to be honest (note that I say "more viable," not "best"). If every living being is infected with the blight, then they don't have to worry about it. However, a significant chunk of the population would die, just as Warden recruits die in the Joining. And wouldn't that ultimately mean the end for humanity if they can't reproduce? On the other hand, trying to do his own Joining on all of the darkspawn seems like a fool's errand, not only because of how many darkspawn there are -- in The Calling he wants to use the Circles to spread the taint around human populations -- but because there is the risk that using it on them will drive them insane, as it did the Mother; insane sentient darkspawn are dangerous darkspawn.
He doesn't want to stop the Blight, upper case. He wants to stop darkspawn from being affected by the Calling. A "Blight" is the massing of darkspawn on the surface, led by an archdemon. His solution is to create the perfect means for an eternal massing on the surface of darkspawn, independent of any central leader and dependent on a political society. Blights right now, are far, far less devastating than they would be if the Architect had his way. The Mother almost kickstarted a blight. It was incredibly lucky that the darkspawn retreated, but all it would take is a new leader, and the surface has a new Blight. The one hope the surface had so far - killing the AD to force the darkspawn to retreat, potentially for centuries - goes away entirely.
There are a number of issues. From a meta-perspective, with all we know about DA, the Calling is good. For the moment, it keeps darkspawn from over-running the surface immediately. The dwarves are close to being exterminated wholesale thanks to the endless darkspawn onslaught. They face a never-ending blight. Without the Calling, that's the future of all life on Thedas. And the darkspawm seem to have a far less harmful effect in the deep roads - above ground, the poison the land and the sky in a way they seem not to poison the cavernous deep roads (albeit they do mutate the life there as well).
The non-darkspawn races have a very, very different calculus from the darkspawn. An eternal calling with archdemons just out of reach is the best case scenario at the moment for Thedas. Sapient - self-aware, critical thinking - darkspawn are the absolute worst. That's an eternal blight in the making, except that there's no way to stop it.
"Sane" darkspawn are just as dangerous. The Architect does not say his followers are "insane". But the evil they perpetrated is extensive. The Architect his darkspawn made their own decision to sack and massacre Vigil. That makes them even more evil. It was his darkspawn that tortured Velanna - they exterminated her people and blamed it on the humans, to then have her massacre them. The Messenger goes around spreading the blight, either killing people outright by infecting them or turning them into ghouls - and we know what happens to the men and (especially) women who are made into them.