That's BS. You just dismiss the possibility of holding onto the joining knowledge without actually doing it, and thus justify countless people dying or suffering for centuries as Grey Wardens. Obviously you'll assume whatever you wish to support your stance on this. In this case being that the people aren't "capable" of remembering something that saves a country. So long as it's conceivable for people to use the higher thought process of remembering something, the Grey Wardens aren't necessary between blights.
I didn't dismiss the possibility of the knowledge being held onto, I simply pointed out that pre-modern societies have a tendency not to take very good care to pieces of information that are thought to be no longer relevant, and that the possibility of it being lost and that the consequences of that occurring would be catastrophic. If you want gamble the future of Thedas on the "conceivable" notion that the knowledge won't be lost, confused, misinterpreted, or intentionally stolen, vandalised, or altered, go right ahead, but I prefer to deal in absolute certainties and the continuation of the Grey Wardens is the most effective and reliable way of ensuring that. There will be another Blight (two actually), no-one knows when it will happen, and even a small chance that the only defence against it might not be there is too great a risk to take. I'm a firm believer in always preparing for the worst case scenario.
Besides which, its been the norm in nearly every society and culture throughout human history that few are sacrificed for needs of the many. In some cultures it's through warfare, in others it's through ritual sacrifice, or slavery, but every culture (even our modern one) willingly and knowingly places the lives of some it's members (military, police, fire fighters, etc.) at great risk to protect the rest of the community and the Grey Wardens are no different. When it comes to Darkspawn, Blights, and life in general, people are going to suffer and die anyway (that's just the way of things) so dying in Grey Wardens is probably a more useful and honourable death than most.
My core point remains the same as it has always been - taking the the high moral ground against an existential threat like the Darkspawn is a luxury that Thedas can ill afford, and the Grey Wardens remain a necessary safeguard against that threat.