In my opinion there has not been a decline in US golfers just look at how many 1st time winners there have been on the US PGA tour in the last few years.
The fact is "everyone" is getting better from India (Arjun Atwal) or Thailand (Jaidee) to China, Korea and think of all those South American players who are playing so well in the USA.
All have one thing in common though, they all want to play on the US PGA tour and obviously $s are the main reason but it is without doubt the best place to go and test yourself.