Not to stroke myself, but I hold a master's degree in Sport Administration and Management, and have written multiple papers on projects such as this (however much larger scale projects, such as Stan Krohnke's $4B privately funded project in LA, and also the Allegiant Stadium and Globe Life Park, so I'm pretty well versed in this topic.
The reason why projections are always amped up is for two things and two things only:
1: Funding
2: Support
It's the same reason why they always project the cost of the project to be lower than it actually will be.
The current trend with new stadiums and other venues is to create a space that can be used 365 days out of the year rather than just the select times per year the stadium has an event going on. It's not like this project is going to be like the Palace of Auburn Hills, where the entire place is surrounded by parking lots.
If you're saying that we shouldn't build a stadium (that has a ton of private backing) that will provide even the slightest of an economic boost on a site that is a complete eyesore to the city, nobody can come up with an idea of what to do with it, just because it could be years, if not decades, until we see an ROI, then I don't know what to tell you, man.