I guess we'll see. My thought is they'll pack the stadium for the first few years and then there will be a dramatic drop off.
NFL fans travel, but those teams play once a week, usually on weekends, and their games are an event. They also can go up to eight years between visits to an opposing team. And most of the fan bases that travel are the northern city teams who love to find a reason to vacation November through March. They'll get fans who are going to Vegas because their favorite team is playing there. Hey, the Vikings play in Vegas, let's make a weekend of it.
After the novelty wears off, I think the A's are going to have to rely on tourists who were already going to Vegas anyway and are dedicated enough baseball fans that they'll go regardless of who's playing. And it won't help that outside of Seattle, the rest of their division plays in southern CA and Texas. The perception is that those fans don't travel like northern city fans do. Though last I recall you were in Austin, so you'd know better than I about that.