@There is enough place for every artist - Stans stop hating others.

1. yes, Madonna is an icon; she is one of the top 5-Mariah, Whitney, Celine, Janet, & Madonna.

2. Your argument is stupid and one that many a Madonna stan has to fall back on more and more these days. One market doesn't necessarily make or break an artist, but an unexplained decline in relevancy, influence, and popularity in one's home market and a market one once dominated does require a hard look at what went wrong. Madonna is a global icon meaning she should slay nearly everywhere on the planet. She doesn't in the US anymore and that is strange. If I use your logic I could say that she is more of a Kylie Minogue these days, a local European artist, or an artist only popular in places where the United Kingdom once ruled or has influence. Even then its not like she is slaying in every European country, just a handful on the markets we tend to count.
3. The US is a huge music market so as far as her career goes it will hardly be irrelevant anytime soon.