Blimey. I didn't know any of that. Unless I've misunderstood are you saying that the BBC axed Doctor Who because JNT walked out? or that JNT could still have been employed by the beeb even after they axed Doctor Who had he not left?
I think I also read once that they approached Verity Lambert as well, but she declined! Not sure if this was during JNT's era or during the wilderness years...
As far as it seems clear, when Impact, JNT's revival of Compact, was scrapped, he made the decision to leave at the end of Season 26 regardless.
If he had been prepared to stay on, he said it would have been unlikely that Season 27 would have happened under his tenure.
However, the BBC did look for a new producer, ultimately in vain. They also told McCoy he had to agree to do a 1990 series, his fourth, to make Season 26 back in 1988, which is why he didn't leave in 1989 like he planned too (he said many times he wanted to do the three-year Troughton-style reign).
When this all fell by the wayside - no new BBC producer wanted to take JNT's place, and no-one could be in place for Season 27 regardless of the ratings, it was decided to try and out-source the series to an indie production company. It should also be noted that Cartmel was head-hunted for Casualty, so Who lost him out-right at the same time too.
Verity Lambert put together was invited to pitch for the show under her CinemaVerity production house, but alas, like all the other UK bids, it failed to get close to an agreement.
As has been noted before, even when Season 26 was airing, Philip Segal was approaching the BBC about potential US production involvement. Where all the UK bids failed, Segal's tenacity won through, despite him alone losing the backing of Colombia, Amblin, CBS, and Steven Spielberg.