I've not dealt with the "disintegrate" aspect yet, but have two ADA tanks set up for two years now. It's very light substrate and I've managed to drop the soil level 50% from water changes and gravel vacs during that time but as far as I can tell it still buffers and "looks the same" as my newer tanks. Also the two brands I've used both raised ammonia so I don't think adding it periodically would be ideal. And a 1/2 cup per year would seem to be closer to the amount you lose than 1/2 cup per water change. Unless you have a really large aquarium or don't change much water. I'd think it's maybe a 1/4 teaspoon substrate lost per water change in my 10 gallon shrimp tanks. My understanding is that the hardness of source water has a lot to do with how long it lasts as well.
Nothing good happens fast in an ecosystem.