1. Your theory could still work with players having all exotics. It would just mean that the exotics that were NOT assigned to drop for them, were sold by Xur. 2. Another way of it possibly working is you have a even chance to drop every exotic, but once you drop one, the chance to get a different one drops exponentially but slowly. So the next few times, the change wouldn't be noticeable. But eventually, when you have 1-2 left, the chances could be nearly non-existent to get them. Just always lowering chances is what I'm saying I guess. This probably isn't true at all, I'm just spitballing here.