In our case, if we developed a full frame camera, it's likely that very few of our recent DA lenses (those designed for APS-C) would be able to properly fill the viewing area of this new sensor. And although we have some wonderful FA lenses, like my favorite FA 31mm Limited, I'm not sure even this lens would be up to the optical challenge.
I know that our engineers have studied these issues and would probably not agree totally with my simplistic explanation. However, I think it's important to understand that going to a full frame sensor means not only having to design a brand new camera from ground up, but likely a new line of lenses that meet the more demanding optical requirements .
