So, let's pretend that the Grey Wardens are disbanded. A thousand years pass, and no blight. And then an archdemon arises.
1. By then everyone will assume that the idea that only a tainted warrior can kill the archdemon is a "legend."
2. The formula for the potion will probably be lost by then.
3. Even if they have the formula, who the heck would be stupid enough to drink darkspawn blood on the basis of a "legend."
4. The archdemon blood needed for the potion will be long gone because no one thought it important enough to preserve it.
I have the feeling that the Dread Wolf could have shown the world a different path, but he's keeping secrets as well. Shame on him for not leading the way to a different solution.
The Wardens do what they do because over time it has worked. They came up with the best solution possible with the data they had. There is a reason the joining is a secret, and unless the Hero of Ferelden can find a cure for the taint, they're probably going to have to keep the joining a secret.
Oh, and while we are waiting for a Blight, who is going to keep the darkspawn population in check? Certianly not the beleagured Deep Roads dwarves who are slowly dying off, and I'd be willing to bet the that healthy and untainted will run rather than risk being tainted in a daily battle with darkspawn. Thus the horde will grow and grow finding the untainted gods will be a surer thing.