But you can't preserve it for future generations, because the location and inevitable impermanence is part of the work. By covering it, you'd be defacing the work to protect it.
Yes, generally we do, and I'd agree for graffiti done in canvas or similar mediums, but I think street art is an obvious exception, since the canvas is the whole street. You could only preserve the artwork by preserving at least the whole building, and possibly more than that.
Are people going to enjoy looking at it less if the building changes (assuming the painted part doesn't)? My guess is that the overwhelming majority of the enjoyment will be preserved.
