And I'm willing to bet the actual numbers of visitors would be in the single digit thousands per year, perhaps even hundreds - basically, just the families of those killed on the plane.
To put it in perspective, Yellowstone gets somewhere around 3 million a year.
I mean, if I was to go visit a 9/11 memorial, the one to the people who fought back would be the one I'd visit.
Still, the Twin Towers can be experienced while touring the Big Apple otherwise. The Pentagon(whatever they're doing there), while hitting all the other DC sites.
Compared to DC and NYC, the flight 93 memorial would be out in the middle of nowhere.
Finally. I wanna know WHO'S PAYING FOR ALL THAT CONCRETE!!!
I'd go for a marble tomb around the size of the unknown soldier exhibit. Be nice and put a maintainable/replaceable weather building over it. The proposed installation is... Tacky.