It's worth remembering that until 1990 we had over twice as many troops in Europe as we have now in Iraq. The average number of troops there in the years between 1950 and 1990 was about 313,000. The number didn't drop below 250,000 until 1992. Last time I looked it was at about 100,000.

(Source) Click on the first link for the spreadsheet with all the numbers.

Whether or not you can make a good case for letting Europe defend itself, and a lot of people certainly could, the fact remains that nobody really cared about it that entire time. In the U.S. nobody cared because nobody was getting killed in hostile action. There may have been somebody grumbling about the expense, but as long as the Cold War was on, it was pretty much overlooked.

In Europe nobody cared because the U.S. was ponying up a lot of cold hard cash.