The action of zooming and focusing with a rangefinder poses several technical hurdles:
-- For a continuous zoom, you would need a way to change the frameline, possibly in major steps (28-35-40-50-60-75-90, for example) or a way to change them so that they represent the actual focal length. With different cameras (often from the same maker) displaying different framelines, the best you could hope for would be an approximate frameline. And that means that composing your photo with any precision wouldn't be possible.
The second method would require a re-thinking of how framelines are currently displayed. Not impossible but it would certainly need a different way than what is in use today. I think this would make it impossible to use this lens on an older camera, and the cost of retrofitting an older viewfinder with a new one might be cost prohibitive.
The framelines for a 200mm lens (a common focal length for the 28-200 and 70-210 zooms on the market) would be the same size as the rangefinder patch and possibly smaller.
You could avoid all of this by using a reflex housing with a rangefinder. But then you simply have a clunky SLR. So why bother?
-- The next decision is whether to use a one-touch zoom (zoom and focus are the same ring) or a two-touch zoom (zoom is controlled by one ring and focus by another). For the photographer, zooming and focusing -- particularly on the long end -- will be more difficult than when using an SLR. Add in a digital crop, and soon you face a real challenge for focusing accurately. An SLR is much easier with a zoom, because you can see what's in focus and what's not. While it's also true with a rangefinder, the experience is different, and minor calibration issues can become much bigger problems in the final image. As well, camera shake that isn't as apparent with a rangefinder when using long lenses is much more apparent when seen through an SLR. I can't see any zoom lens for a rangefinder going beyond 135mm because of problems with focusing, camera shake and framelines.
-- Next, the company wants to make a profit. As with most of these "why don't they make" queries, what's possible, what's practical and what's profitable don't always end up on the same page. As Leica has learned, catering to a small but vocal group of users/complainers doesn't ensure profitability. You would have to allot money for research & development, as well as marketing. And then sales would have to recoup these costs, plus profit, so the price for a limited-production product serving a niche market probably would need to be much higher -- sharply higher -- than what most people expect to pay for a zoom lens.
-- There also is the mindset of rangefinder users who don't want a zoom because it's generally accepted that single focal length lenses are much better performers in terms of sharpness and distortion.
-- The SLR is ideally designed for a zoom lens, especially a continuous zoom lens. I can see no situation in which we'll ever see a range of zoom lenses for rangefinders. It's not impossible ... there is very little that isn't possible. But from a company's standpoint, it's probably not practical or profitable.
Short version: I'm not holding my breath waiting for such a lens. I'll use an SLR instead. Even so, it's nice to dream about such things!
