He says that the demons of Hell should not siege war on Heaven for multiple reasons: Heaven is way too fortified for even an attempt to infiltrate. Belial knows the power of God and that is should not be tested, especially so quickly after their recent failure. He continues to say "what is the point?". We are in Hell with some potential opportunities, why go risk angering God so that it can all be taken away. His go to phrase would be "things could be worse". The demons could still be chained to the fiery lake with no where to go or nothing to do. Why go out of their way to take over Heaven when they all know that there is no chance of winning.
Things could be worse
Ultimately, Belial thinks they should stay put. Wait it out. He wants to stay slothful and try to make the best out of the shitty situation that they've all been put in. Even though his words are only wrapped in garments of reason and don't hold any actual meaning, he has a point. If I would be down there with them, his idea would have gotten my vote. I would actually be more comfortable with Belial's and Mammon's ideas combined. The labor/attempt for redemption and keeping to myself sounds like a way better idea than pissing off the most powerful being in the universe.