Yes - of course - but there are always at least 10+ other diamonds on the Comfort+ upgrade list - and that usually means they bought relatively late too (generally high fare class) and at least a few will have the MQD waiver, corporate travel priority, and/or reserve card. I've been as low as #15 on the Comfort+ upgrade list as a diamond (although on average I end somewhere around 3-7 on the list after everything clears).

The routes I fly most are ATL->West Coast (PHX, SEA, SFO/SJC, LAX) during peak hours. There's usually 100-130 people on the first class upgrade list. There's enough people that that a lot of times they board PREM, then Platinum (separately), and then SKY. By the time they finishing boarding sky, the plane is 75% full. So even getting a Comfort+ seat is tough. I fly middle seat in Y very frequently. It is what it is.