10,000 - 15,000 patrons sounds pretty optimistic for a first-time festival. But if they get it happening then great.
I went to Oregon Brewers Festival this year and that was a fantastic event. Their website says 70,000 beer lovers attend over the 4 days. I guess the Friday would have about 15,000 which was busy, but definitely comfortable. The key will be having enough beer tents/vans to ensure lines aren't ridiculous at any of them.
I'm interested to hear more about it.