They'd be able to finally rest, knowing that there were no inferior races polluting reality.
This would probably drive them crazy, but they weren't thinking that far ahead. Eliminating all inferior races is their sole goal in life, but the rank and file Daleks have probably never really sat down and thought about what would happen if they succeeded. They just do as they're ordered.
As for the Supreme Dalek, it's been documented in the past that Dalek leaders are short-sighted and uncreative, and that's why they need to keep Davros alone in the first place.
In "Genesis of the Daleks", the 4th Doctor asks Davros what he would do if he had a virus that could kill all life in the universe. Davros answers that he would use it, because exercising such power would make him a god. Davros is obviously a megalomaniacal lunatic, and he created the Reality Bomb.
The Supreme Dalek's other advisor is Prophet Caan, who is insane. He knows in advance that the plan isn't going to work, and in fact he helped create the plan in order to force the Doctor to destroy him and the other Daleks. ("Journey's End")