Now we have an official timeline as to when that will happen, and it actually started yesterday. As of July 29th players will no longer be able to make in-game purchases for PC, Steam (2.0 only), iOS, Android, Amazon Android, and Apple TV. You will be able to buy in-game items in 3.0 via Steam.
On September 30th, the PC, iOS, Android, Amazon Android, and Apple TV versions of the game will be removed from app stores. This day will also see the end of Community Team reviews of new Toy Boxes for all systems. You will be able to download Toy Boxes however for Consoles, Apple TV, and Windows 8 and 10 until March 3rd, 2017.
On January 3rd, 2017, in-game purchases will no longer be available in the Windows 8 and 10 versions.
This all culminates on March 3rd, 2017, when the Apple TV and Windows 8 and 10 versions will officially go offline, and no longer be playable at all. At that point, all Disney Infinity online features will also be shut down.
It's important to note that the various console editions of the game will continue to be playable, with "full access to figures and play sets", just without any online features or multiplayer. The same goes for Steam versions of Disney Infinity 2.0 and 3.0, which will also remain playable save for online features.
We knew the end was coming, but it's still a little sad to see it all laid out like this. Still, it is good to know that you'll still be able to play the console games offline for the foreseeable future.