Wait - I forgot to mention two important facts. Not disagreeing with you but these may be important facts. I initially had a confirmed seat in C+ on the first flight (LGA to Boston) - let's call it DL 999 (made up flight). I
was #1 of maybe 20 on the upgrade list for DL 999. My inbound flight was delayed, and so I used Facebook to SDC to the later flight - mind you, this was only 90 minutes before DL 999 was scheduled to leave. By the time my inbound flight landed, I ran to the gate and saw that DL 999 had been delayed by about 30 minutes.
The gate agent refused to let me SDC (despite one C+ being available) and put me on the standby list, suddenly
I'm #8 on the standby list for DL 999... I get that the standby list and the upgrade list are different, and also, I don't know my fare class but I paid about $100 for C+, so definitely a low fare class - thoughts?
Also, as to your "everyone was a Diamond or Platinum" - which may very well be right - wouldn't most Diamonds and Plats pay the extra $14-$20 to book C+, and therefore, in theory at least should have been able to SDC rather than SDS?
Looking at 12/4 LGA to BOS for fun - the difference between Main and C+ ranges from $14 to $20.