Two sections that I wanted to address.
Long-term success for any football program (and football is driving the bus here; any talk of other sports is filler) depends primarily on winning. Winning your division, winning your conference, winning MNCs, etc. The notion that somehow a move to the SEC will transform these programs from middle-of-the-table in the Big XII to conference and national championship contenders is stupid. A&M and Missouri can't win their own divisions in the Big XII, much less the conference championship.
The issue as I see it for A&M and Missouri, is that if it looks like the Big12 is either going to be perpetually locked in as the "Texas and Its Bitches" conference, or better yet a "Texas and Its Oklahoma Sidekick" conference, what's the difference? If they have no prospect of long-term success anyway, why not go for the conference that shares its revenues more equally? The middle of the table in the SEC might be a more lucrative option than in the Big12, even if it does nothing to enhance their actual on-field success.
If the issue is stability, and it's assumed (or just feared) that somebody else is going to one day pull a Texas A&M or a Nebraska and potentially pull the conference apart, or that the Big12 has no long-term viability, then Missouri might as well make a grab for some stability while they can. That applies to the Big10 or the SEC, neither of which are going anywhere anytime soon in any kind of reasonable scenario. Moving now might be better than later if there are concerns about all of the chairs being taken if they don't time the Big12 dissolution correctly.
So, the idea is that they move to the SEC West, and the magic power of the SEC transforms these programs, the same schools run by the same people, into programs that will consistently beat LSU, Alabama, Auburn, and LSU (not to mention Georgia and Florida)? Finishing fifth or sixth consistently will make these schools worse at football, not better. Are MSU and Ole Miss better football programs just because they are in the SEC? Hard to argue when they are consistently at the bottom of the pile looking up. Finish fifth, sixth a few years in a row, which is where A&M will be next season. Recruiting declines because recruits want to play for high-profile winners, not SEC losers. Attendance sags because a lot of fans will find better things to do than watch LSU kick their heads in on Saturday afternoon. Coach is replaced because he can't win. Lather, rinse, repeat. They become Ole Miss and MSU v 2.0. They maybe make a few million more in TV revenue once the SEC deal gets re-negotiated, but that's about it. Whoop! Gig 'em!
Man, I know that Arkansas is the best school in the SEC West that nobody (outside of Arkansas fans, obviously) seems to fixate on, but that's just mean. Did the Cajuns and Creoles each get their own LSU? Actually, Arkansas might benefit more intangibly from the A&M (and possibly Missouri) additions, because I never get the impression that Arkansas is the game that gets the blood really pumping for the other SEC West contenders (LSU maybe? Is the boot a big deal? I get the impression that Saban and the last decade of Auburn-LSU while Alabama was down have made it basically a three-way hate fest that leaves Ark as the "eh" game), and that's probably partly due to geography and the SWC migration. A&M should help that quite a bit, and Missouri provides another nearby rival.
More seriously, two things on this piece. You are describing Missouri's future in the SEC West, not necessarily Texas A&M's. A&M should be positioned as being no better or worse than Arkansas, right up there with the titans of the West depending on where they are in any given roster maturity phase. Good coach/mature roster = up top with whichever two of LSU, Alabama, Auburn, or Arkansas happen to be great at the time. Bad coach/immature roster = fighting with Mississippi State, Ole Miss, and Missouri to be the least bad.
The second bit is that, if A&M and Missouri join the SEC West, Auburn moves to the East. Therefore, they only have to deal with Arkansas, LSU, and Alabama as the divisional titans, with rotating battles with Auburn, Florida, Georgia, and Tennessee.