It's not a hard & fast 'number' for every single account holder. (look up the threads or google Customer LIfetime Value, CLV)

They have some way of determining desirability of customers - presumably including spend, services used, products engaged with, etc. - and use that when decided who gets what.

It might also be that AA has asked that low spending Plat holders get turned down, since they are subsidizing that luggage benefit.

Anecdotally it does seem like something has changed, and AA might also have contributed to the changes too.