A few things:
I don't fully understand the physiology or psychology behind why if makes such a big difference, but having pacers completely changes a race. Elite athletes even use pacers for training runs. It also makes a big difference having someone to chase or on your heels and even knowing you are on pace for fast time. None of that was the case today, so end all of the discussion about time.
Rupp has several miles towards the end where he was in maintenance mode and probably could have easily run a minute or 2 faster even off of a slow first half had he made his break earlier and/or pushed towards the finish line. A fast race for Rupp would have been 2:04 or 5; I and sure he figured there was nothing gained from running 20:07 or 8.
Brett could have been a pacer, although I doubt it, but I don't know, I don't care and can't understand why anyone one else cares. From what I understand, the race did not hire pacers, but I don't think it was against the rules to have a private pacer. There are quite a few things about the race that clearly suggest Brett was probably not a pacer; since when do you use a pacer to run slow?
Chicago on paper was not weak field. You had a few guys who hadn't run fast in the last year or 2, but that's not so far in the past to suggest they still weren't capable of running fast. You also had a couple of athletes who entered late and perhaps they thought they could steal a victory even though they probably were not pointing to Chicago. To win a WMM like Rupp did, you really have to be 100% ready and training for the specific race. Also, there were several athletes in that race whose career has been on the roads. For some reason, they can run for 2 hours at a 4:40 pace, but when the pace is slow starting out or if someone throws in a couple 4:30 miles at any point, those athletes struggle or even drop out. Obviously the problem is not endurance, perhaps there is a muscle memory advantage to having run a fast 10K. I see this phenomena a lot in Olympic marathons where you have 2:02 athletes who can't break 2:10. I would not be surprised to see some of those same athletes who ran Chicago, run much faster in their next marathon.
Don't be surprised if see Rupp back competing again as early as January. In which case, if you are not going to run 2:05, or 6, you are better off running 2:09.
A different athlete. I was super impressed with Jordan. She ran 2:20x and it looked like she could have run a minute faster if she had someone to chase in the last couple of miles and/or someone on her heels. There are only 3-4 women in the world have consistently run faster.