Managenent has said this also. The problem is that the trials needed to show a survival advantage clearly enough for FDA to permit a label claim are almost impossible when a disease has a typical survival of 4 years. For starters, it is unethical to withhold Jakafi for more than a few months given its dramatic QoL benefit over any other treatment (except perhaps in minimally symptomatic patients, but that brings up other issues). To show a survival benefit, a trial should last around 150% of the typical expected survival--6 years. That is too long, and is too costly to do. By the time it's done, you have issues of remaining exclusivity on the drug.
There is a long and depressing history of cancer treatments that were touted as having a survival benefit on the basis of no or insufficient evidence, and were eventually subjected to voluntary trials of sufficient power. FDA is correct in making such claims difficult to put on labels--but it does sometimes give medical troglodytes cover for ignoring progress.