The Coke ad at the end was a stroke of genius. Don finally got his game back.
My theory also is that they Roger and Marie getting married, created the opportunity for Don and Megan to reconcile, as Don realizes he can no longer just run away from those he hurt anymore. And then together they raise Don’s kids in New York, despite Betty’s wishes. And then after the non-compete period is over, they leave McCann and form, Sterling Draper Olson Holloway and Associates, after they buy out Joan’s firm, and land Lear Jet, with Pete, as their first client, and the band is reunited.