Hearing above 15kHz

Although you can't hear a pure sine wave test tone above 15KHz you probably can sense 15 KHz roll off. A 10 KHz signal (for example) which is not a sine wave may have a wavefront steeper than a 15KHz sine wave. Based on my own (68 yr) ears I think that the hearing sense involves steepness of wavefront. I can think of no other explanation for what I hear.

Correct. There seems to be little evidence that tweeters that go accurately up to 30 KHz and more are actually necessary. In the end all you gain are bragging rights. However, a better tweeter response within the audible range (up to 20 Khz) will of course provide benefits. (There is a lot more to a better response than just frequency range, however simple specifications lend themselves to marketing hype)

I feel your pain. I too don't really hear much past 15K.

That specific tweeter may not be as useful to you as it would be for a younger person, but all of the speakers you mention have their own "sound" that includes the rest of the spectrum. When you listen to these speakers you are either going to have a preference for them over other speakers or not.

Every product you buy, whether audio or otherwise, probably has a feature or two that are somewhat superfluous for your needs. That shouldn't keep you from buying it if the other performance aspects are perfect for your needs.

Good point Mlsstl. I cound probably disconnect the ribbon tweeter in the Helicon and not notice any difference. But if the balance of the speaker sounds great, that's all that really matters.

Perhaps, as mentioned above, we can sense quick rise-time or high intensity sounds at or above the fixed pure sinusoidal limit of our hearing.

I see an experiment unfolding!