We spend too much time talking about tweeters

I do it too. I'm guilty.

Just saying.  85% of the sound out of a speaker if not 95% is not in the tweeter, but the marketing people have us talking more about them than anything else.
Once you hear a good tweeter you'll talk about it also. The Magnepan ribbon comes to mind. 
I agree with everything Duke said. Kind of to add to my original thesis, how many times have discussions about entirely different speakers been focused on their tweeters? Is it Be? Diamond? Ceramic? AMT?

It’s as if the tweeter itself defined the quality of the entire speaker, and goodness knows the cost on the high end can be exorbitant for such little devices!

The tweeter gives everything its definition, presence, air and dimensionality. Texture, tone, instrument voicing and placement are all defined by the tweeter. Maggie’s ribbon and B&W’s Diamonds excel in these areas...the tone deaf or the high frequency challenged need not agree. Worst tweeter designs I’ve heard are ribbons...sand paper comes to mind!
" So yeah, if we think we can hear to 20k, and tweeters are crossed over at 3.5k then that’s 16.5k of a 20k range... "
What this says is true if one thinks only in Hz, but music is more commonly measured in octaves.  Music covers ten octaves and 3.5kHz to 20 kHz is less than 2and 1 octaves./2