We were dealing with a hypothesis in which the timeline is altered significantly, and which posits an either neutralised or defeated Britain.
As such, the assassination raid that killed Heydrich doesn’t take place, because that was launched from Britain.
That being the case, it is a given that (absent evidence to the contrary) Heydrich would have lived.
By the time of his death (in our actual timeline) he was already the Fuhrer-designate, and had been so confirmed by about three days. The signs leading to it were apparent within about three month’s of Rudolf Hess’s flight to Britain.
As such, I chose to hypothesise that Heydrich would have lived.
As a personal view on Heydrich, I think him to have been a genuine human monster. However, Adolf Hitler openly admired him, Heinrich Himmler feared him, Canaris was very very wary of him. All of that comes from various accounts I have read over the last 25 years or so.
Granted, this thread is dealing in hypotheses, however, Heydrich surviving into the 1950’s at least is certainly valid within the nature of this thread.
As such, he is very likely to have become Fuhrer, once Hitler dies from whatever cause, though the Parkinson’s Disease is the likeliest.
Regards, Uyraell.