It wasn't that crowds were sparse at WWTBAM at DCA. Almost every show there would be a good sized crowd - the problem was they were mostly DL AP holders - many at all the shows in a day. I was at the last show of the day back in July and indeed they pointed out some who had been there for every show!
While obviously pleasing to the AP's who went to the shows, it wasn't drawing people in.
Now at MGM (and WDW in general) the story is much different, as you don't have the percentage of AP holders, nor the average number of trips made by an AP holder.
Still, with the prime time show itself a fading memory (unless they ressurect it again for sweeps), you never know. However this is hopeful (from millionairetv.com):
COME JOIN MILLIONAIRE AT WALT DISNEY WORLD® JANUARY 13 - 16TH
Millionaire will be taping at the Who Wants To Be A Millionaire-PLAY IT! attraction at the Disney-MGM Studios at Walt Disney World® Resort in Florida. We're offering travel packages which include tickets for a Millionaire taping, discounted resort accommodations and Park Hopper® tickets to all four Walt Disney World® Theme Parks.