Ace Combat Infinity is shutting down on March 31, 2018. You won't be able to purchase anything for the game starting on January 9, 2018, and you won't be able to download the game starting on March 1, 2018.
Farewell, Infinity. You helped carry this franchise to the next generation.
Ace Combat 6 and Assault Horizon work on a peer-to-peer system solely backed by Xbox Live / PlayStation Network / Steam. As long as Microsoft's/Sony's/Valve's servers are up, those multiplayer modes will stay up because they're basic peer-to-peer connections.
Ace Combat Infinity uses a combination of peer-to-peer, PlayStation Network, and Bandai Namco's server-side technology to provide us with the game. You notice this every time you turn on the game and it gives you the "Loading player information" message boxes. If Bandai Namco's servers for Infinity go down (which they are on March 31, 2018) then the whole game -- campaign, free mission, free flight, multiplayer -- will be unavailable.
I don't know why people expected any different. This is how all online free-to-play games work, they're a temporary thing. And @MyNamesFurii there is no offline functionality in Infinity. Even when you enter a campaign mission, it does the whole "Uploading player information" deal, you need to have an internet connection to Bamco's servers to do anything in Infinity.
Obviously, if they decide to suddenly push out a patch to let us play campaign forever, then this will change. But right now, this is the way the game works per its free-to-play nature. Everything will shut down on March 31.
I'm actually really sad about infinity shutting down. It was the first Ace Combat I played and it's the reason why I became a fan of the awesome series. I'm excited for AC7 but I'm still sad about infinity.