The effect which this will have on people will depend upon whether you are an "elite on one airline" person, or a person with the potential to be elite on more than one airline.
As for me, I am only elite on one airline currently (AA--and only as a successor to TWA). Extension of my elite status will certainly encourage me to fly AA for another year, since I get perks on AA that I don't get elsewhere, and may not be likely to get elsewhere.
Persons with possibilities of elite on another airline may shift to get the elite level on the other airline by the end of the year.
However, I believe that there are probably far more people in the first category, who will remain loyal to AA because of it, than there are in the second category, who will see it as an opportunity to get elite on a second airline. But that is just a guess.
Djlawman