I'm intrigued why more fixed-mount lenses with a simple, high quality zoom aren't offered more often. Right now, the best bet for fixed-mount lens with a small, high quality zoom are some "cheaper" APS-C Leicas or the high-end point and shoots like the Sony RX100 series or the G1X II. Fuji has the X10, X20 bodies that do this as well, I think...
I think the reason why is manufacturers want lens pullthrough dollars, so the added cost / hassle of making it modular in as many body designs as possible is more profitable in the longer term. Just guessing, though.
You may be right about all that, though it's perhaps ironic that for some (doubtless a tiny minority) part of the appeal of mirrorless bodies is the ability to use lenses that aren't being made any more.... Plus, you have to wonder - yet again - at Sony's RX10 and the slightly newer Panasonic equivalent, with their supposedly excellent longish zoom lenses, which must make a lot of potential customers wonder why they should bother with Sony's and Panasonic's other mirrorless bodies.
As for an earlier point you were making about mirrorless needing to be small, that may end up being true from a marketing perspective, but for some of us (who knows how many - I suspect few, though) mirrorless has appeal independent of size (I prefer EVFs and the absence of AFMA-causing mirrors, for instance). But if you do want to keep it small, you can compensate quite a bit for loss of reach, while keeping decent-size sensors, if you boost resolution. The extent to which you can crop on a Sony a7r when using a very sharp lens such as the Sony/Zeiss 55mm 1.8 is remarkable. And if you don't insist on speed, Sony (85mm 2. and Nikon (film-era 100mm 2.8 E) have shown that you can go fairly long while remaining remarkably small and light.