Similar arguments being advanced here to those which were made in favour of keeping more than two fast jet fleets. The flashy and expensive Typhoon and F35 both spend most of their time doing stuff that could easily be done with less capable platforms, but we need both types' capabilities in reserve for certain scenarios. The cost of using them inefficiently in the meantime is evidently less than the cost of running a third fleet.
Reading between the lines of the expert contributions above, my take is that we shouldn't really be operating both C-17 and A400M: the former was a stop-gap for the latter. However the users have come to value C-17 so much that it's unchoppable. And the A400M is both new and European, so in policy world it's unchoppable. So unless either user or policymaker budged from their positions, the C-130 was going to get chopped. The user appears to have concluded that C-17 was worth keeping over C-130, and I don't think many would argue.