Something that became very clear to me many years ago is that thereminists do not agree on what constitutes a good instrument. There is no agreement on configuration, linearity, design, sound or playing technique.
There is some level of disagreement (thank God) in all musical communities but because there is no established tradition for either theremin making or theremin playing, everybody has their own ideas.
This is further complicated by the fact that most people come to the instrument as adults with their own preconceptions already formed about what a theremin should be. I have often heard people who have never even owned a theremin hold forth at great length about how a theremin ought to be built, played, and sound.
The reason people can get away with this seems to be because, as I said, even theremin "experts" disagree and there is no foundation for the teaching, playing or building of the instrument that we can point to as basic.
IMNSHO this will not change because the instrument itself will always remain marginal, on the musical fringe, appealing only to a relatively small but dedicated group of enthusiasts (dat's us folks!!). Why must it remain marginal? Because as a device for the making of music it is severely limited, extremely difficult to play, and it is highly unlikely that it will ever be possible for anyone to make a full time living from it.