If BYU's goal is a National Championship, the AAC and MWC isn't the way to go.
I look at it this way:
If BYU's goal is to get in to the 12 team CFP, the best bet is probably joining the MWC. Boise State is down so we shouldn't have much competition in the 2nd best G5 conference. We should be able to dominate the MWC like we did the WAC. We have the name brand to get the ranking, and then we can hope we end up ranked higher than the AAC champ.
If BYU's goal is to be the best G5 we should join the AAC. We could settle it on the field and if we win the AAC there is a good chance we end up with a CFP bid.
If BYU's goal is to win another National Championship and have a relevant program that is respected in a way that we as fans think it should be, we should stay independent or join a P5 conference. We aren't going to be able to recruit a National Championship level team that could compete with the teams we'd be facing in the Playoffs. Likely every G5 champ will be one of the lowest seeds going in, so we'd need to knock off a top team, then another, and another to get to the Championship. We aren't going to be able to do that consistently as a G5. As much as you want to hope the new CFP evens the playing field like March Madness, it doesn't. One or two players on a Sun Belt team can do enough to cause a couple of upsets in basketball. Football is a different animal. You need a ton (literally) of talent in the trenches, speed everywhere, top notch D, and excellent QB play to have a chance, especially if you have to do it 4 times in a row (probably on the road). There are outliers of G5 teams that were good enough, but most of them are tied to elite level coaching. I love Kalani but don't consider him Urban or Petersen level elite. But I do think if Kalani has the right personnel on the field, he can get it done.
The only way BYU will be able to build a National Championship level team, or even a team that is built to make a deep run in these new Playoffs, is by staying Independent or joining a P5. It's all about the players (just ask Saban, Dabo, Urban, etc) and if we are officially a G5 and there is a P5 up the street, I don't see how we compete. We've been able to recruit with our psudo-P5 status, our ESPN contract, and our awesome independent schedule. That all goes away if we join the AAC or MWC. BYU needed the exposure from last season and Zach. If BYU can build on it with a special season this year, I like our chances at landing more talent than ever, cementing ourselves as a legitimate option over Utah to non-LDS/non-Utah recruits, and getting more national respect than ever.