The problem is for a festival to even get that big takes 3-5 years and even then you need a huge appeal like camping which you can't really do at an inner city site.
Edmonton even with major league soccer would sell probably 40-60k tickets a game MAX so even then you have 30-40k seats empty. It just doesn't make sense for the city.
The idea behind it sounds cool and great but logistically and financially it doesnt.
Supporting your case here.
I think you vastly over-estimate the upside for soccer attendance in Edmonton.
FC Edmonton has averaged 2,500 people a game. Last year was up to about 3,500.
The Sounders are the only team to average more than 25,000 people a game.
I really don't think you are going to get 15 times (30,000 - 35,000) more people going to games because the quality of the game improves from the second tier to the first tier of soccer in North America.
If anything selling soccer here is like selling hockey in the southern states. Always going to be a niche market.
All of that to agree with you that a 70,000+ capacity stadium for soccer in Edmonton makes absolutely zero sense.