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I don't get the infatuation with 21mm. For tripod, landscape use, it might be great, but for everyday use, 21mm seems too wide. I use a 28mm for everyday shooting, and I find I need to take great care since it's quite wide. I'm not sure $3k is something I want to spend if I only use it once in awhile on a tripod.
For most, It would definitely be a specialty camera if an RX version with either a 21 or 24mm lens was ever introduced. The 21mm focal length has a certain importance in regards to Zeiss, not to mention there is also a history of specialized, fixed lens super wide angle cameras. One such camera I shot quite a bit alongside a Rollei TLR and that was a Brooks Plaubel Veriwide 100 with a Schneider 47mm lens covering 6x10 (yes, actually wider than 6x9) offering a 100 degree angle of view. It had a Leica 21mm optical finder atop.
The RX1 with it's 32-35mm (actual focal length is somewhere in there depending upon if one uses distortion correction or not) is the general purpose camera. For myself, I find 28mm a tiny bit too wide for general use.