Whatever the merits of RF's vs. SLRs may be, there's one incontrovertible advantage to SLRs: You're not likely to see much in the viewfinder with the lens cap on!! I have both types, and even my Leica M4-2 can take a perfectly exposed shot of the inside of the lens cap, unless it's removed. Of course, "perfectly exposed" is open to debate.
As for the framelines, they automatically adjust when you change lenses. And there's a control on the front of the camera to try other frames to see what would happen. Someone mentioned altering the lens mount slightly to change the frame from, say 50mm to 40mm. Hey folks, this is Leica - you don't play with success.