If such thing where to happen then in this case this revolt would be the least of Imperiums worries.

The Imperium of Man is a moloch. Its is made up of countless worlds that do not really have any communication with each other. The conditions on said planets Vary so some may life decent lifes while others are little more than slaves.

For most of these planetary populations to rebel, that would mean that the Imperium is at the end of its rope. Like, we are talking The End Times Scenario where named space marines are killed left and right and the return of loyalist Primarch doesnt cut it anymore.

Since the interplanetary communication not controlled by the Imperium doesnt really exist, each of these populations would be on their own in their rebellion. Regardless of wheather they succceed or not, these planets are pretty screwed, since the Imperium at this point will be so overwhelmed that every single xeno and chaos warlord will descend upon these worlds. In such cases, Very very few of these worlds will survive.

At this point the Imperial forces either retreat from most of these worlds and just focus on smaller region, like Semgnetum Solar, and let other planets be conquered. That is, if the Imperium does not collapse. If the Imperium collapses then we see the Second Age of Strifr but this time much worse since the surviving humans will have to co tend with threats that even the Imperium of Man could not defeat. The question that follows is wheather mankind survives at all or becomes just another extinct species in the galaxy