Apples and oranges. "More room throughout coach" failed because of exactly what it was: throughout coach. Thus it was nto possible to charge more for it to only those people that wanted it.

That's why this time it's not "throughout coach" (except maybe on JFK-LAX 321Ts?), it's only a certain number of rows of coach. And most passengers (those who are not elite) have to pay extra to reserve seats there.

Therefore, AA does not have to "cancel" MCE if it's not working perfectly, they can just change the number of MCE rows per plane if it's not big enough (or if it's too small).

And keep in mind that AA was the last airline to do something like this (a subset of coach with extra legroom, as an upsell). It has already succeeded at UA (Economy Plus), it has already succeeded at DL (Economy Comfort), and in fact it has also succeeded at some of the LCCs too.

So all the problems that MRTC had (it wasn't scalable to more rows or fewer rows, based on observed demand, it was a concept that no other airline had proven before) are not applicable to MCE.

The only things that MRTC and MCE have in common is both were AA and both gave extra legroom of some sort. That's where the resemblance ends. But to the bottom line of the airline, it's what the other facts are that are relevant. (The airline cares whether it's a feature they make money on, not whether it's an extra legroom feature! And from the income perspective, MCE is totally different from MRTC.)