1. Most, not all, freighters are well used airframes before they enter service, typically as a conversion. A lot of freighter's typically fly less than a passenger aircraft, so they can use the airframe at the higher end of the time/cycle frame and are available for maintenance more readily.
2. For freight the blended concept looks like it would have to carry a whole lot of different sized containers - not very functional.
3. The idea they should try it as a freighter first to establish some sort of track record seems to suggest the freight pilots are some sort of guinea pigs. Which reminds me of a DC-8 freighter crash long ago out of Salt Lake which killed the 3 person crew. The AP reported that "there were no people aboard".