J.V.
It would seem that you're right judging by the fact that there is no hint of a conversion programme for the A343. Of course, there isn't one for the A333 either but this is probably more to do with continuing pax demand and holding values.
I guess the one variable that might change the outcome of your analysis would be the value/lease rates of the respective aircraft. If pax demand continues to hold up values/lease rates of the A333 while the advent of the A350/787 cause values/lease rates of the A343 to collapse, the acquisition cost advantage of the A343 might outweigh the doc/operational advantages of the A333F. Interestingly however, at present, appraisers value the aircraft pretty much the same so don't appear to have factored in the A350/787 threat yet (either that or they think that the A350/787 will hit both types equally).