Wearing my businessperson hat, I think basic economy actually did work quite well to extract more money from people -- it's really tough to live out of a backpack that fits under the seat in front of you for more than a few days, and that leads to a decent number of people buying up to Main Cabin. Some people will live out of a backpack but don't want to be stuck in a middle seat. The fact that AA is currently not selling many domestic BE fares is puzzling, but I have to think that it is related to the elimination of change fees. I think a permanent elimination of BE would end up like More Legroom Throughout Coach -- folks will not pay enough of a premium to fly AA.
AA launched BE much earlier than B6 and at the beginning, the difference between BE and Main was substantial. Perhaps AA learned that they were yielding less doing that as they had to massively overbook BE buckets but no way of knowing with certainly since AA's Rev Mgmt team is truly worst in class... even well before Covid, they kept pricing F lower than J on 3-class MIA LAX 77W flights.