$5,000 is for 10 guests.
Very different than $8,000 for 4.
And even if that were the case, them "selling out" tours means nothing. The number of tours is quite limited: 1,000-2,000 people per day. Certainly their new FP product would need to appeal to far more people than that.
There are no where near enough people willing to spend that money on FastPass, the program would be a massive failure.
Hell, you could get 6 FastPass+ reservations 90 days in advance for only $50 per day per person while staying club level; that program failed miserably according to multiple sources.
Real talk though, it's not going to cost that much money lol. Worst case is likely $1,000 per person for a week.