It is a great idea, and a huge financial windfall on many fronts.
The cost is huge, having to book a stadium, with all that money going elsewhere. Rather, they can get free labour to build more assembly halls, all with donated money, and then the halls become an asset for the Watchtower property empire. Then all donations for running costs go to them as well. Doing a link up saves money, as it means less travel costs required to send the GB all over the US.
They could continue with 3 days, but they may as well just do 2 days, as Friday was always the lowest attendance as many people cannot, or cannot be bothered getting the time off. As people get less interested, it will become an embarrassment to see the difference in attedance, and to avoid that why not just do weekends.