I'm not sure that is true. I have been doing this for many years now. Used to be very easy because O was often avail. The response from AF was to decrease the O availability, not from DL to do anything about it. And we know that when there is a "loophole" of sort, DL is quite prompt at correcting it.
I think the system works as intended. No particular loophole. The issue now is O is seldom available. On my last ~2 years of W flying, I have not been able to find upgrade available. I have an EF alert set for my next flight. I booked over 3 months ago... Still nothing. At this point, I know I have more chances of op-up than upgrade ;-)