People seem to forget that running 2:22, or 2:19, is something that takes a lot of commitment. In addition to all the things Keith listed off, the other often unsaid piece is the finances.
I'm not an expert, but my impression (and a little bit of googling seems to bear it out) is that 30 years ago, people came out of college with debt that could be easily retired with a part time job.
Now, as both tuition and college debt levels are rising faster than inflation, it is more and more common for people to have levels of debt that are not easily paid with a part time job.
So, the purist idea of "seeing just how good a runner I can be" is getting harder and harder to do. If you were back in the 70's or 80's and could easily focus on running, and pay your bills with a little work on the side, great, why not take a shot and see how good you can be. Now, with the pressure of school debt, the increasing trend towards needing to get a post-graduate degree in todays job markets, and all the other things that make modern life rather complicated, is it that surprising that people accept a PR of 2:2x high, and move on?
That's what I did- I sometimes daydreamed about really focusing on running, and I probably had an outside shot to hit the 2:22 mark, although it would have taken a 5-6 minute PR (pretty unlikely). It would also have taken postponing medical school (or actually, probably forfeiting it- hard to tell the admissions committee: "Thanks for the offer of entering your class, but I want to see if I can run a bit faster over the next few years, so that in my next marathon, I have 2 miles to go when the winner finishes, instead of 3 miles...). I doubt my then girlfriend, and now wife, would have thought it was the most brilliant plan either.
In short, the 2:19 standard isn't destroying american marathoning (nor is it helping), it's just that people are being forced to move up the big decisions in their lives, and one of the first things to go is trying to become a sub-elite marathoner.