As someone who counts himself as a reasonably serious runner, and has participated in parkrun here in the UK on a regular basis for over ten years (and has no vested interest in it), I can honestly say that it has done more for a lasting legacy in terms of running, social fitness, community engagement etc than anything the Â£9billion we spent on hosting the Olympics has done.
I know many top UK runners (including Olympic and national standard runners) who take part, as well as countless members of local running clubs at all standards. But also I love that my partner who uses it for general fitness turns up regularly, as well as my 74 year old father and 6-9 year old nieces/nephews at different runs near where they live - all of us with different reasons for attending, but every one of us getting something positive out of it.
If I want a fast road race then I'll enter one and pay accordingly, but for me, parkrun lets me test my fitness versus previous weeks/years, use it to try different tactics for other races - and the best thing about it (other than being free) is that you can decide on a Saturday morning if you want to partake without having to announce or commit in advance.
I also volunteer occasionally (say if I'm racing on a Sunday), and I love it that I can get involved in supportive way in the running community. It's never a chore, and it's a great social occasion to chat to other runners about all things running.
The knock on effect of a healthier community surely doesn't need spelling out, but there's also an immediate economic benefit as all the cafes/restaurants in the park vicinity do a massive trade for a few hours. Perhaps you should look to engage them if you're looking for initial sponsors. I know a few cafes at parks near me in London who would go out of business if it wasn't for parkrun.
I'm convinced that once the US sees the benefits stack up against the initial costs and naysayers, then parkrun will thrive.