Exactly. No way should they introduce yet another different airlifter.
However if we need a medium/heavy air lifter in the future the A400m will be the only aircraft being produced so we may have no choice.
More C-17's may cost more now, but they will get flown less and last longer. This means that you save money on buying replacements. Also it helps reduce the workload of all the other air lifters, so the money goes a long way.
It would be the safest option. You can never have enough transport aircraft however not enough aircraft can cripple the military.
However rarely do you actually need the ability to carry a single 50+ tone load. So you could just split the load between two C-130's. Aircraft in the C-5 and C17 size always claim that they can carry heavy armoured vehicles but they never do in real life. Thats what ships are for
This is the argument that the government will use not to buy more C-17's. In the local region the C-130J's are more than capable i guess.