"The point of conventional instruments is that once you learn them - which takes years - you can instantly express almost any musical idea using all the possible degrees of freedom available on that instrument."But the degrees of freedom of conventional instruments are severely limited compared to what is possible.
Let us also recall that every "conventional instrument" was at one time not only unconventional, but even radically new. The piano, is itself only a few hundred years old. I'm sure when it was invented there were some people who argued against its use and that one should instead stay with "conventional instruments", which then did not then include the piano.
I strongly recommend a talk[1] by Jordan Rudess, who is widely considered to be one of the greatest living keyboard players.
In this talk, Rudess discusses and vividly demonstrates the greatly expanded possibilities that innovative keyboards bring to the table.
Novel instruments that somewhat resemble conventional instruments like the keyboard are only the tip of the iceberg of music interface possibility, however. There are plenty of novel music expression technologies that don't have even the remotest resemblance to conventional instruments, and allow ways of expression that were hardly imaginable a hundred years ago. Things like whole body position tracking, which allows you to make music through dance.
Of course, mature musicians like Rudess who've spent their entire lives learning and practicing on traditional instruments will be unlikely to switch to something radically different, as they'll be starting from ground zero on those instruments. But others with less to lose will be more open to learning something completely new.
It's impossible to tell which novel instrument will become the conventional instrument of tomorrow, but it's very likely some will, because that's how we got all of the conventional instruments of today.
[1] - https://www.youtube.com/watch?v=8h-TsqoSWgo