As the article mentions, this is probably to prevent other camera makers from doing this. As the article mentions, the huge issue here is cost. How much would users be willing to pay? Would you pay $1000 for a digital back for your F100 or buy a D5100 for the same cost. In terms of functionality the D5100 is more advanced than the F100. Its a lot easier to build DSLRs from the ground up than design a digital back within the constraints imposed by a film SLR body.