Since Dominion is closing...

You're missing the point. Riot doesn't have the resources to give Dominion the attention that it needs to make it a game mode that more people would play. If they took it down just to put something else up, it's likely the same thing would happen.

They're just going to have temporary new game modes from now on, like URF and Hexakill. They'll probably bring back Dominion for a few weeks every year for old time sake. Thankfully we'll still have ARAM.