You talk about mezzos and contraltos. You're probably right about them. TRAINED voices, as opposed to ordinary people's voices. Fine. But THAT is not the point.
Virtually any American who can carry ANY tune can sing 'America, the Beautiful' or 'Columbia, the Gem of the Ocean', or even Lee Norwood's tune.
Francis Key's poem is remarkable, even to this day -- but to insist that the tune to which it has been set is even marginally acceptable as a national song, well, that's just bizarre.
No offense to you, of course.
And I'm not a TRAINED singer per se, never studied voice, just an old choir singer. Folks we get that walk in off the street have an octave plus. Of course to some degree they're self-selected 'cause they're trying out for choir, but plenty of 'em can't actually sing.
But not because of lack of range, that's not the problem. Too much vibrato (in the women) poor tone (in the men) and inability to duplicate pitch are the major problems. And let's not even start on inability to read music . . . . thank goodness my parents made me take piano as a kid.
Just randomly riffling through the 1982 Hymnal, almost any traditional hymn is going to have a range of at least an octave, and usually more (I'm not talking about the pop trash that finds its way into just about every hymnal nowadays and seems to constitute most of the output of Oregon Catholic Press). "America" is over an octave, has the same range as "Jerusalem the Golden" which is imho the hardest thing to sing in the entire hymnal (come to think of it, both tunes have almost the same awkward leap).
If you don't like the tune, that's cool, but don't claim it's difficult to sing, because if you learn the trick it's easy. I think ballfield organists who double as accompanists for sopranos tend to pitch it too high for the average person - they always do for me!