Not being retired, I wouldn't know about that part, and maybe in Latin America it's comfortable living on an American pension or Social Security check, but here in Western Europe (possibly excepting Portugal), the cost of living is higher than in the USA, and anyone living in Germany gets their SS taxed in Germany, full rate and no 15% exclusion, either.
Besides, Democrats abroad, as long as they are U.S. citizens, can still vote absentee, campaign locally (Democrats Abroad, Germany, of which I am a member, is very active, from the Ostsee to the Bodensee) and make contributions up to the legal limit. What contributes more to mediocrity, an educated professional lending time to public Democrats Abroad events on the streets of London, Paris or Berlin, or someone parading around some American college campus with a hand-painted sign saying "tax the rich" or some other equally original slogan? Sure, there will be some Americans abroad who tune out, and are comfortably retired in a cozy bungalow in Cuernavaca. Most Americans abroad bear no resemblance to that image.