So there is the benefits of utility, safety, and acquisition of power. These are just the three advantages that they can tell for now. There could be more advantages to be gained in the future.
But they don't need more advantages to decide to steal the Mindscape. In fact, they don't need the three advantages to decide to do so either.
Even if they had no use for the Mindscape, and even if the Mindscape is not a danger to them and can't affect them in any way, they still won't mind stealing it if they can just to rob someone else of power.
Just the option to be able to eliminate a future threat or competitor is enough for them to decide to steal the Mindscape. Now that they have three legitimate reasons to do so, there was no way Legion wasn't going to try to do it.
Earlier, they had said that they would be content with the Mindscape if it could help them acquire the power they need. Apparently, they had lied about that.
