All I can say is, don't believe everything you read on a bulletin board while going to a funeral.
In my area, they're still having the convention at a larger venue. So perhaps they will still do so for many unless they see some specific reason to do otherwise. The cash JWs bring into the areas with larger venues is probably a big reason for the bigger stadiums and the like to offer them a good deal. I don't know about you, but I'd try hard to make sure 6,000+ people kept coming back to fuel my city's economy every summer.
Assembly halls would be a good choice, but the stadiums and all that bring with it the potential for press, and therefore, a bit of free advertising. Thus, the potential for more recruits just reading it in the papers or finding out about when JWs go to their hotels, restaurants, and the like. There are a number of good reasons to do it that way, reasons that could--could--be worth the cost.
--sd-7