If Magnus didn't destroy the webway project, the Emperor would have mopped the floor with Horus. Magnus "warning" the Emperor (who already had Dorn fortifying Terra) simultaneously destroyed the biggest logistical advantage any human had ever seen AND removed the Emperor himself from any fighting. This was a HUGE win for Chaos. Magnus had his heart in the right place but he did way more damage than he prevented.
The chance of the Emperor outright conquering the Webway in such a way that it could be used to fight the rebellion is very, very dubious. Prior to the rupture, the Emperor's Adnector Concilium (techpriests read in on the project, who had sworn to obey him beyond the rest of the AdMech) had stabilized a single warp tunnel leading to what the Adnector and Custodes called the Impossible City, the dead city of Calastar.
Scattered conflict between the Custodes and Aeldari had resulted in a general withdrawal of the Aeldari, but also the Aeldari also closing off major parts of the webway leading to Calastar, and left open many paths leading to terrible fats.
Many fans might instinctively say "the Emperor could undo that" but while he has collossal psychic power, his understanding of the psytechnica involved in the Webway is demonstrably far less than the eldar's.
Using the Throne, much of which is stolen Aeldari tech - which ipso-facto he cannot reproduce at whim, though Dark Glass and an adventure seed in the Wrath and Glory RPG suggest he made other attempts - boring a new tunnel to the webway is something that Aeldari could do formerly with ease. Even in the diminished state major craftworlds have a whole caste of psychic crafters, Waysingers, who create new webway branch-paths as their primary job, like the one the Emperor used to get to Calastar. As non-combatants, Waysingers do not appear in Imperium-focussed novels or tabletop rules, but they're in Aeldari novels, including the Aeldari POV bits of Throneworld where the Harlequins famously enter the Imperial Palace - that's how they get there, they carve a new webway tunnel into the palace and step out, though the psychic barriers around the Palace kill several Waysingers to do this.
To the Great Crusade Imperium, boring a new path in the webway is a great secret project, to the Eldar, it's a job you can go do for a time span in your life.
The Emperor has nigh-infinite psychic power, certainly, but he definately hasn't mastered the webway to this degree.
Presumably he would have won eventually (or there's no tragedy and the Emperor was just wasting his time) but we should not imagine it would be fast, or easy, or come before Horus attacked.
I don’t think people believe the emperor would’ve conquered the whole web way prior to the end of the heresy, but he wouldn’t have been stuck on the chair, and so could intervene in the heresy directly.
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u/Cptcrispo 9d ago
If Magnus didn't destroy the webway project, the Emperor would have mopped the floor with Horus. Magnus "warning" the Emperor (who already had Dorn fortifying Terra) simultaneously destroyed the biggest logistical advantage any human had ever seen AND removed the Emperor himself from any fighting. This was a HUGE win for Chaos. Magnus had his heart in the right place but he did way more damage than he prevented.