The Japan game was a stinker. I don't think it's a larger referendum on Gregg. Even when the results weren't there in qualifying, the US didn't really lay stinkers like against Japan. They had the best xG margin in the Octo. Sometimes, they just couldn't make their control pay.
I don't think the exclusion of Ream or Pefok from this particular camp is a terrible thing. I think both would be on my World Cup roster right now, but I'm not sure I would have called them for this camp either. Because I feel I know what I need to know about them. I know what Pefok is good at. I know he's not a natural fit in many ways for how the USMNT plays and that if he's the 9, he'll NEED to score because he's not going to do other good things the way that Jesus and even Sargent can. That doesn't mean I don't call him, because, frankly, I don't think the US is good enough to have a 9 that does other good things without scoring. But I can see how you'd want to give Sargent, Pepi, and Jesus a chance to show they can score and do the other good things in this camp.
I think Ream is maybe a tougher exclusion for this camp, because he really is a step up in distribution from anyone but Brooks. I probably would have called him in when Musah got hurt because Musah is really the key to more modern Gregg tactic of using the MF to advance the ball by shuttling. But I think that Gregg, like me, probably thought LDT could do that in Musah's absence. He didn't against Japan (in his defense, I'm not sure Musah or Nagbe, or Pogba or prime Jean Tigana could have done that with Wes and Adams having dumpster fire games too).
And that's really what I took from the Japan game. The back four wasn't good, but I think the real issue is that the team will never play well if both McKennie and Tyler don't have it. The tactics just won't work without it. In the World Cup, if that happens, maybe you put Pefok and Pepi both upfront and eephus it up with some Route 1, though I can imagine what the Gregg critics would say about that.