I doubt it on that scale.
The Imperium itself is huge and there's no way a Primach can be everywhere at once.
Plus the Emperor never intended for any of the Primachs to actually rule/govern the Imperium itself. That was one of the reasons Horus betrayed was because he and the others felt left out as the power was given to the High Lords of Terra.
In any case the return of any of the Primachs would give the Imperium a major morale boost and give it a better fighting chance against the gazillion threats besieging it but don't forget that at the same time you could also bring in the Daemon Primachs to balance things out anyways.
Ok that makes sense so it's essentially another Armageddon where the Tyranids are locked in an endless war with the Orks......is it a good thing or a bad thing? Cause that would mean that tendril will never move past Octarius and the number of Orks charging into the fray will be focused on that instead.They destroyed all the Tyranids on the planet, and while there is still Bio Ships in orbit the warzone has been pushed into what the Orks would consider a victory. Before Ghazghkull arrived the Orks were on the verge of defeat, but now it has become an endless battle. This is exactly what Ghazghkull is aiming for. He did to Octarius as he did to Armageddon. His victory is an endless war across the whole galaxy. Besides after the "victory" Ghazghkull leaves to find a new place to create this endless conflict again.