Leica's optical rangefinder module is much more expensive to build than an AF system, which can be bought from a subcontractor.
Just leaving out the multiple-frameline feature wouldn't make it that much less costly, and reducing its complexity would harm Leica's reputation (for example, the short-base rangefinder in Bessas is quite good, but a Leica with the same module in it would be considered a huge step downward by Leica fans.)
So, a fixed-lens Leica RF would still be quite expensive -- its cost would have to be that of an only-slightly-simplified camera body plus an only-slightly-simplified lens (since Leica fans would still expect a top-performing lens even on a fixed-lens camera.) That combination likely would put its price below that of an M7 plus lens, but more than an M7 body alone.
And since it would cut the company out of potential profit on the future sale of lenses, there would not be a lot of business incentive for Leica to introduce it.
That's my take, anyway.