I take those points about promising "magic", but I do think P&W thought they could (hopefully still can) do it. It wouldn't have been good for them or anyone else if things were otherwise. That, of course, doesn't make the issue any less grave. As t43562 says, this will doubless be a significant redesign, which could involve some effect on the whole platform.
It was always a 'big ask' (sorry, I don't like that expression either), but that's how boundariesare pushed. One of the consequences of this concurrent development is that so many aspects of this program are now stalled.
There is no plan B, so this had better work. I always understood and was disappointed by the spec of this thing. My hopes for the final product are not high anymore.