I think it will be. It pretty much has to be.
I don't agree with this. Rocket League is a game with a ton of hype and excitement, Xbox is well known for online multiplayer popularity and looking at any available sales charts we can see that multiplayer games do incredibly well on Xbox in general (and always have done).
Sure, its releasing 7 months later or so but I think Rocket League is something people would be willing to double dip on to play with their Xbox friends and also new people will want to play given the popularity of the game and multiplayer on the platform. Its an easy to get into and enjoyable game, its also what $20? Ark is doing pretty well launching 6 months later in Game Preview at a much higher price.
I don't think online community is going to be an issue for Rocket League on Xbox One, perhaps if it was a larger scale multiplayer game then more questions arise but given its 4vs4 max there will be a lot of people playing with their friends in parties and doing matchmaking etc, it'll be fine and if it doesn't launch on Games With Gold it can always go on it later
Plus exclusive Halo/Gears/SO skins!