The thing is, Lissandra wouldn't want the Watchers to consume everything. She would much rather that they be her tool to conquer the Freljord, which, of course, didn't exactly work out for her. If they destroyed everything, there wouldn't really be any land left for her to rule, would there? Lissandra wants power and control; the Watchers want destruction and nothingness. Two very common goals for villains are actually pretty contradictory when put in context.
As for Malzahar, who knows. I think we might get some insight onto what the Void thinks of him when we get a Voidborn update sometime. Especially if Malz is still the one who brought Kog'Maw into Runeterra. And if so, will we find out who Kog's "daddy" is already? We know that the Watchers made the rest of the Voidborn species, but, the Voidborn do reproduce, and I have the feeling that Riot originally intended for Kog to be from a special and probably fairly intelligent race of Voidborn. Perhaps Malz has a plan concerning them, if so? Of course, this is all wild speculation, as we have very little up-to-date lore on the more "grounded" Voidborn and how the physical Void inside the rift operates.