So if you where new to the game, and someone posts this, would you be happy that that plus 10 blue would be auto dismantle? Yes, I am aware you say it should be a toggle, but bungies habits with toggles are really bad.
If you had read my whole post (This I'm more understanding over though as I didn't reiterate it several times like the others) I said the option should be a thing once people hit the soft cap. IE Forsaken how it was, 500 I think? If you don't know what a soft cap is that's okay, it's the point in the game where blues stop dropping (engrams) at higher light then your current gear, which then makes them useless.
Make it like D1 with greens. They're irrelevant at your level? Auto-dismantle.