I think more of what is killing the 747 market accept for freighters is the 777. If there was a 777F there would be no 747F sales. Costs are much lower on the 777. Why, 2 engines instead of 4.
Mr. Boeing has been quietly working a two engine version of the something that looks a lot like a 747, but with 2 engines for a couple of years. Yes they looked at it a long time ago, but no engine existed that could satisfy engine out criteria.