The whole episode is fascinating to me in that Bloomberg is a reasonable quality news organization and something obviously convinced editors there to stand their ground in spite of no obvious (presented) evidence. I agree though that absolutely no proof has come to light which makes me seriously question the whole thing.

There is abundant evidence that these type of hardware inserts exist and are being deployed. Why do you think it isn't happening?

This is the same argument that was used for decades to suggest that NSA was not hoovering up all voice communications though.

It is technically possible. There is adequate motivation by capable parties to do so.

Apple would be negligent if they did not make a serious effort to validate the hardware.