A 1x converter would move the lens forward and prevent the lens fromfocusing at infinity. Bad idea. Canon did make a DSLR model (one of their very first, before the D30) with a glass window between the lens mount and the mirror box to prevent sensor dust, but it turned out the glass was prone to breaking during lens changes, at which point the user had a much worse problem than a few dust bunnies on the AA filter. Between the cut/puncture injury hazard from the glass splinters and the incidence of glass shards jamming and damaging the mirror mechanism and shutter, the idea was scrapped. And now you know the rest of the story...