I don't work in an industry where anyone travels heavily, but every millennial I know (who does travel at least 5+ times a year) doesn't really care about any travel program. A lot of them will have 1 cheaper CC for the bag benefit on the airline they travel the most, but otherwise it's all about cash back CCs and Rakuten, and I know a lot of people that are pretty tuned in to how to maximize cash back. I think there is a small potential in doing better with airline miles (if you know that tricks of the trade), but the biggest problem with airline loyalty is that it's often unclear what the real benefits are.

With a sub $100 a year CC, you can get a free check bag and early enough boarding. Once you do that, what is there to gain by even getting gold?

MCE seats? Meh. unless you are long legged, it just doesn't make that much of a difference to a lot of people. Cramped is cramped whether your knee-to-seatback clearance is 2 inches or 4 inches. (and, the chances of sitting next to people of a somewhat bigger size in MCE seems like twice as high, IME, vs being farther back).
Being on an upgrade list? Meaningless. Except on short regional hops, you often have to have like 300K LPs to have any hope and chance.
Companion upgrades on the upgrade list? Even more meaningless since IME it's impossible since AA hardly ever releases two seats at a time. So you just get skipped. And skipped, and skipped, as they trickle clear 1 seat at a time.
International lounge access? Most people would hardly ever get to use such a thing, and let's admit it, lounge access is more vanity than value.
More than 1 free checked bag? Such a rare person who checks even 1 bag, let alone 2 or 3.
Bonus miles on flying? Perhaps this is the truest benefit, but, it's hard to justify loyalty for an extra handful of devaluing miles.

Personally, as someone that is now in my 40s, I am only loyal to AA because they are the only airline that flies to my airport and that's convenient for me. I do often reach for 2 miles vs 2-3 cents cash back because I still have some faith that I'll get more than 1.5 cents per mile of true value towards trips I want to take. But I've stopped caring about my airline status, deleted my LP tracking spreadsheet, and am downgrading my AA CC. I have no real interested in travel awards CCs either.

(That being said, I feel like I get crazy value in my hotel loyalty program)