Actually, that's not entirely true. In a statement today, SEGA advised that the 360 version will still be available in certain "bundles" going forward, and as you'd expect it'll still be re-downloadable for anyone who owns it already.
Here's the statement:
"We're excited to share SONIC X SHADOW GENERATIONS with you this October and see a new generation cherish this game as much as we do! On 9/9, the original Sonic Generations will be removed as a standalone title. But, we've heard your feedback, and want to help preserve the game."
"Therefore, Sonic Generations will still be available via bundles of other iconic Sonic titles on Steam and Xbox. Existing owners of the title will still be able to download and play, as well. Lastly, we are happy to confirm that mods on the Steam version will NOT be affected!"
Despite the "9/9" mention, Sonic Generations is not currently purchasable via the Xbox 360 store, so your only chance to buy it now is either as part of these apparent "bundles" or via the disc version if you have a console that supports it.
Of course, what SEGA really wants you to do is buy Sonic X Shadow Generations this October, which features a complete remaster of the Xbox 360 version along with a whole bunch of new content. We'll leave that up to you!