Hearing above 15kHz

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!

My hf hearing at 50 years is long since shot due to working on oil platforms for years before health & safety ruled.
My speakers do 40kHz and I can still enjoy ( notice not saying hear) the benefits of super & ultra tweeters.
So who knows?

I am a few years over my half century. I tried out Townshend Supertweeters and could'nt hear the slightest difference, except an unpleasant screech with my ear against the supertweeter at maximum volume. Luckily, Max Townshend is a gentleman and took them back with a full refund

Just because you cannot hear tones above 15K does not mitigate against having loudspeaders that have response at least an octave above this. First, a 15K roll off will cause phase shift within the audible range. The audibility of such phase shift has been debated endlessly, and argument can be made that the phase shift within the crossover and driver implementation of most speakers may swamp this, but let's just say that less is better. Second, high frequency tones create beats, or formants, that fall back into the audible range. For example, a 19k tone and a 20k tone, when played simultaneously will generate a low-level difference signal at 1k. Again, the audiblity of this phenomenon is debatable, but certainly measureable. It sometimes amazes me that some measurement objectivists say that we are imagining things when we hear differences that are not measureable, and that we are imagining things when we hear things that are measureable.