Fortunately, there is simply more than one "America's sidearm".
I knew that some people would mention the SAA because of its success as a design in its time, and how there are many imitators. However, it had a much shorter service life, and technology wise, it was more on the obsolete side, much sooner than the 1911.
This is part of the key to why the Colt SAA is one of America's handguns. Probably the first one truly thought of as America's handgun both in this country and globally. It's short service life does not discount it because it continues to be made and used today. It's clones and descendants are numerous. Though other guns were used, like the Schofeld or Remington, none others were as influential, as copied, or as emblematic (rightly or wrongly) of America as the Colt SAA. That a lot of this is due to John Wayne and Clint Eastwood only adds
to it's place as "America's Gun".
If a fella wants to include the S&W M&P they run into problems. The first being Colt. Colt and S&W were neck and neck for the DA revolver market in the U.S. till the 1960s or so.
From 1900 or so till the 1980s the U.S. was a nation of wheelgunners. They were much more popular than semis. But they used both Colt, S&W and Ruger guns. Not just the M&P.
If the argument is for all American made DA revolvers then you have to explain away the European Webleys and Enfields, etc. The argument can be that more American made DA revolvers were sold than any other. But I'm not sure that cuts it.
If you argue for the snubby revolver, than I agree.