I've personally experienced a case where there were C+ seats open, but there was no SU inventory. It was on a DTW-LAX flight I booked a couple weeks before the flight and the seatmap showed a couple open C+ seats. It would be rather absurd for DL to go through all the time and expense of implementing a separate fare class and upgrade bucket for C+ if SU always equals W.
Given the ability to now greatly goose your miles/MQD earning by booking KL marketed DL flights, I don't think it's that big of a trade-off to lose the ability to upgrade to C+. Do you prefer the previous situation where KL marketed TATL flights earned based on spend, but you could get an upgrade to C+ at time of booking (because C+ was not yet a separate fare class on TATL routes back then)?
Seatmaps mean nothing. There could be a ticketed passenger without a seat assignment - I see it all the time (e.g., 3+ seats on a 739 at the gate instead of the 2 normally blocked - invariably someone is cleared from the standby list into one of the seats before any upgrades are processed).
If you saw a W fare for sale on delta.com and did not get an upgrade, then that would be different. But I have not seen a confirmed example of this - ever. I would love, love, love it if Delta did capacity controls on W inventory as it may mean that I would actually get a C+ seat every once in a while - but as far as I can tell they do not do it. Maybe they will start doing it soon but as of right now it doesn't appear that they do (at least in any sort of systemic way).