This is just not true (maybe it was true at one point). Maybe my experience is anecdotal but I feel like Delta definitely does revenue/inventory management on their Comfort+ seats now, or at least reserves some for day of departure - or it least it looks like they do anyways. For a flight I have next Monday, I can go and buy a new Comfort+ ticket via the Delta website right now (or at least I could yesterday when I posted - it looks like it's sold out now). This has also been the case in the past where I could not find a buy-up offer (and did not get upgraded instantly as a diamond). And I know come Monday morning there will be 2-3 Comfort+ seats available for "day of" upgrades (but I usually do not get them as I do not have a reserve card nor do I have an MQD waiver so I usually rank pretty lowly even with a relatively high fare class).