Leaving viewfinder parallax aside for the moment, there are a couple of things that RF users get used to doing for street shooting and wides. Take a look at the C/V 21 and 25mm lenses and notice the levers on the focusing rings. These act as tactile focusing indices, and you can know about where you're focused and, once you're used to the lens, make adjustments before you even lift the camera up to your eye. In addition, the C/V 25mm has click stops on the focus ring at 1m, 1.5m, and 3m. Use them intelligently by feel before you raise the camera, and with a properly chosen aperture the DOF will cover you. You might also want to calculate the DOF for the 21mm f:4, even wide open. There's a lot of room to move there, and combined with the feel you'll get for the focusing lever, this is not a huge problem, even with an auxiliary finder and no rangefinder.

Lots of street shooters who "stalk" their shots don't have the camera up in their face for more than a fleeting second before and after the shot. This style of shooting benefits from wide angles and what I think of as a manual pre-focusing lever. Several RF shooters I know scoff at the idea of waiting for a camera to focus itself when they can have it done before the camera is raised.

Obviously, this may not be the way you prefer to work, but it's a traditional one that's been very successful for a number of people, and is not obvious at first glance to someone who's not used to 35mm rangefinders with focusing levers/tabs.