So, lets go through that list:
* China Cargo/China Eastern : Another existing 777F operator, as well as 77W. Doubtful.
* Emirates : Another 777F operator, who has just ordered 2 more and has 77W's being converted. Don't operate the 350 yet by the way. Not likely.
* Etihad : Another 777F operator. Highly unlikely, they just aren't in a position to order anything extra. Only possibility would be if they converted some existing orders for 350-1000, don't think that involves ALC,
* Air France : Their cargo fleet consists of precisely 2 777F. As long as they operate their fleet of 77W's there is fleet commonality just as much as there would be with the 350's. Highly doubtful.
* Air China Cargo : Another large 777F operator who also operates 77W. Not likely.
So following this logic, why did any of those airlines order A350 passenger aircraft when they could have ordered (more) 77L/77Ws instead?
Well besides EK and EY, none of them had the 77L in a passenger configuration, a lot of them had 777-200's but that's what the 350 was aimed at replacing. EY quickly got rid of their 77L's when the 77W could do the job of their 77L's. EK have only recently ordered the 359, as well as 789 and that's because they finally worked out they need something smaller than a 77W or 380. Now most airlines who own 777F's aren't in any hurry to replace them, they just aren't that old, they are a great freighter and even the oldest ones have at least 10 years left, most likely with their original owners.
So where is the logic in replacing an aircraft that is economical, great at it's job, with lot's of life left in it, if owned probably paid off, with an expensive new aircraft that does the same job while granted using less fuel. How long will it take till the shiny new investment is actually cheaper too run when taking into account ownership costs, or higher leasing rates ?
Of course the same will apply for a 77XF as well.
Things would be different if you were talking about ordering 20, 30, even 50 aircraft, the economies of scale start to make a difference, but it is highly unlikely that anyone would order that many 350F or 77XF freighters in one hit at this time.