Well, guess I mite as well toss my own experience into this ring. I've shot an "M" body for more years than I care to admit, along with just about every brand and model of digital to come down the pike. I've done the Nikon route, D1 & D1x. Then even did a Kodak 14N with it's Nikon mount. You want a long zoom? Find yourself a 70-200 VR-G used, and you will look no further. That lens, in my opinion, is an absolute classic in front of the right digital sensor. Fantastic lens. Far superior to the Canon 70-200 offering, especially wide open. Yea, it's big, but once you see what it can produce, the size and weight are easily tolerated. Real pretty. Try one and see for yourself, and you will save yourselves a big pile of bucks over the cost of a newer model, and give up NOTHING to them in image quality. With out looking "sterile" like so many newer lenses of a more modern vintage....
As to the 21 Leica "M" lens, well, I have two right now. My personal favorite is my 48 year old 21 SA on my M8. I also have the pre-ASPH 21 'El. For what I shoot, both of these lenses positively rock on full frame film (M6, M6 TTL, M7) as well as stunning on my M8. Hard to beat any of them, though again I personally prefer the look from the older lenses, even if they aren't as sharp. Sharp is way over rated also, in my book. You think that Cartier-Bresson gave a hoot about sharp? His magic was the geometry, not the lens resolution.... In fact, I don't believe I have ever seen a really sharp Bresson image in a print.
I have absolutely NO inclination what so ever that all of the Leica "M" lenses won't function just fine on a full frame "M" body. What they don't fix with software as long as your lenses are coded you won't mind one bit. And if you do, use one of the many software packages yourself to correct whatever doesn't please you. The game in digital now, as it was in the old film days, is half capture, and half post processing. That is why Jack and Guy go to such great lengths in their workshops to teach both. One without the other is only getting you half way there.....
The 21mm focal length is very much about the field of view and perspective. With the signature "look." You want ultra sharp? Use a 90 f2, or my personal favorite for sharp, the 24mm ASPH. But then, I shoot mostly people, and with the wider angle lenses. That 24mm ASPH is probably the sharpest lens in the M lineup as far as overall resolving power stopped down one or two stops. With incredible micro contrast, I mite add.
For myself, I see no point at all in waiting to buy a lens that you would use now if you can afford it. How many good images are you going to pass up today, just to buy into someone's marketing hype telling you to just wait and see what is coming down the road tomorrow? 200? 500? Hard to say, as that same hype doesn't tell you a darn thing about what will actually show up. I figure if it does the job I need to do today, then I have a winner with whatever is here right now. I try and take care of my needs today, and let tomorrow bring whatever it will. One thing I can guarantee you, whatever comes tomorrow is going to be three things. Different than you expect today, a whole lot more expensive than what you can pickup a used 21mm for right now, and way too late in coming to help you make beautiful images over the next year. Just my own opinion, others probably feel differently about it.