That's the thing though: we always knew the ending. Horus rebels. Horus fights the Emperor. Horus dies. The Emperor is interned upon the Golden Throne. The Legions of Chaos retreat to the Eye of Terror. Guilliman rebuilds the Imperium and forever alters the Legiones Astartes into the Codex Astartes.
A stand alone book could hardly devote the page space to go into such detail as was displayed in "Angel Exterminatus," since the ascension of Fulgrim into a Daemon-Primarch would have only been a short part amongst the almost dozen of similar stories -- some more important to the central story (illustrated above) than others. "What If" scenarios like the one you are describing in detail wouldn't be possible without the fleshed out mass of books we've received thus far in the HH series.
So, you're kind of arguing my point for me by such conjecture.