When will we ever hear turntables demoed fairly?

I'll tell you one thing if you promise not to tell anyone. There seems to be an objective reality. People with equally excellent hearing will hear exactly the same things. They may and likely will have to a various degree different sound preferences. Most people with great hearing will never become audiophiles because they can't tolerate this garbage coming out of any speakers driven by any amps from any source. I know a few people like that, they appear to be able to hear things that I cannot, two of them are professional classical musicians.
As for choosing turntables, this would not really be a big problem for me, even less so if I were prepared to spend $10k or so on the table/arm. I would easily accept a great sound even if it is not the very best possible at this level. In my case it would be even easier because I only consider British turntables. If cost were almost no object I would consider everything starting with those $10k for table/arm and would go from there.

I would take away another message, Bruce.

You can get good sound, but it takes diligence. Beware.

Lots of retailers can show you good sound. Some can show you the difference between a cheap system, a good system, and a great system. Small differences may be elusive, and some retailers may try to promote the highly merchandized / high margin units, but with some diligence you can get mighty good sound. Perhaps not the ultimate - that's the work of decades of trial and error.

****So my final takeaway on my original question is that you can't achieve any real comparison that means anything objectively.****

Your premise is not quite true.   There is value in comparing tables with different arms and carts on them.  There are characteristics of sound that are primarily influenced by a turntable's speed stability which can be perceived no matter what arm or cartridge is on it.  Rotational stability is probably THE most important aspect of reproduced sound as this affects the rhythm and feeling in music.  We tend to focus on frequency response related issues and "detail", and not as much on speed stability.  Good speed stability affects soundstaging and our perception of detail and overall tonality as well.  I think you are correct that a thorough comparison cannot be made with different arms and cart, but paying close attention to speed stability is a darn good place to start.

Yes, Frogman, right on. There is another aspect. Platter mat and record interact, it's like kind of sandwich. Can one hear the differences of this? Experienced good ear can. I myself probably cannot but may still have a feeling that something of that is not quite correct if it is the case.