Maurice Martenot confronted this question back in the early 1930's and came up with the ideal solution; an instrument that would offer the musician both the standard keyboard and continuous control.
After consulting with some of the most celebrated musicians of his day, Martenot came to the conclusion that the theremin, as a precision instrument, was simply not practical because it was too difficult to play and too limited in regard to what could be done on it by even the most skilled virtuoso.
Here we are 80 years later and the situation does not seem to have changed. More people play the theremin today than at any time in the past (and there are now more theremins in the world than ever before) but the instrument remains, for the general public, a musical curiosity - a novelty of the past.
Yes, it the passion of a small group of hobbyists and dedicated enthusiasts (of which I am one) but it is not taken seriously by the music world, nor is it likely to be. Why is this? Because, as Martenot said long ago, it is too limited and too difficult.
There are theremin festivals and events all over the world, and like-minded people come together from far and wide to celebrate the instrument but when you hear the music they are making on it, you quickly realize that their love and appreciation of the instrument far surpass their ability to play it.
I don't think this is a bad thing, in fact it is a good thing. The theremin has given many people an outlet for musical expression that they would otherwise not have had. The quality of the music they make is unimportant in the light of the personal satisfaction they get from making it.
"I think it would be wonderful if everybody could play the theremin as well as I do, but they don't." Clara Rockmore