We do indeed need this aircraft.
Development of airframes and engines takes time - sometimes a very long time. At least they have the engine and are not faced with a hole in the ground.
Sadly, the fully developed and fully mature C-17 is no longer an option. And while the C-17 is only about 20% bigger, it has nearly 100% greater payload and almost 200% greater range with the same payload, about 100% greater cargo volume, and yet similar short austere field performance.