That is a very fair point. My primary airline never oversells J, so I always make the assumption that the front-most cabin on any flight is never intentionally oversold. This may be an incorrect assumption for me. But I eventually got MSP-DCA in J.
However, SFO-MSP was further delayed, and DL suggested I rebook through JFK.
So I was rebooked from SFO-MSP-DCA to SFO-JFK-DCA due to the mechanical issue, but only confirmed in Y on JFK-DCA due to lack of space. Fine. I asked to be added to the upgrade list since I had paid for J. SC agent couldn't do it, and asked me to talk to the gate agent. GA couldn't do it, and added a note. Still unacceptable. That allows a Silver on cheap Y to get the seat before me, since I have no status, which is absolutely ridiculous.
So I tweeted them.
Twitter said I should pay $169 to sit in J. And again, here we have my issue. I'm not some "buy-up" pax. I literally went to DL.com and bought a first class SFO-DCA round-trip ticket. But they said I "purchased Comfort+". No, that's just where the GA in SFO seated me because it was the highest available cabin. I paid for F.
I'm glad I got that sorted out, and really glad they found space to confirm me, but multiple agents said I'd be below all medallions trying to upgrade their Y fares, and the Twitter agent said I "purchased Comfort+".
This kind of customer service is why I don't have a preferred airline in the US. I buy F on whatever's most convenient, because they all seem to suck like this. I do not want to have to argue about what cabin I paid for.