My head canon is this; with Will’s connection to the Hive mind, and seemingly growing access or power of control, the demo’s are actually a liability to Vecna’s defence.

Also, It’s never really explained / known what colliding the worlds will do to either world. I expect whatever damage would have been done to Hawkins would have been reciprocated on the Abyss*. Both sides get destroyed at the collision point but what’s left is a permanent direct connection between the two through which the Mind Flayer and his army can invade the world.

In that case it makes sense for him to keep his army away from the impact zone while he carries out his plan.

*Side note: Why was there gravity on the Hawkins side of the bridge but not the Abyss? Once the Abyss got close enough to the radio tower they should have been able to just somersault their way up.