I think that's definitely an option, especially for urban areas. It's not as if they really need smaller centers to coordinate the preaching work, that's all gone by the wayside.
I have a couple of friends who attend megachurches. If they don't attend, they can watch online. Preacherman always is on one of the big screens, the choir on the other.
They love it. Being part of a big congregation (if you can really even call it a congregation, at that size) is part of the thrill. Occasionally there's a big event (picnic, hot dogs, music, etc.) in a city park.
It doesn't appeal to me, but as they always remind me, being a JW ruined me for all religion. (Yes, it did.)