Not sure how you'd strike back in a meaningful way if the provinces for the main Empire aren't on the map. Nowhere to march any armies.
Sure, you can take back the provinces that China invades, but that sounds pretty darn unsatisfying.
How else do you figure you could strike the main Empire? Maybe they'll throw in a few events. Even if they have you a real big one like assassinating the Emperor, you won't really get to see the fallout. The best I can see happening is an event that says "Oh boy, you got them good... You should see it. Chaos. Pain. They're real sorry they messed with you. Honest."
This is precisely the problem with this half-baked approach to China: what we're basically getting is "Sunrise Invasion" with a handful of diplomatic flirtations to stem the tide. At least when Sunset Invasion came out, it made sense that you wouldn't be able to strike back, as the Aztec homeland was across the Atlantic; but China is so darn close!!!
Sadly, it looks like Paradox is putting on its pointy hat, grabbing its staff, and telling us that we shall not pass.
But there could still be hope...
Maybe we'll get lucky and some devs will go rogue after support for CK2 formally ends? They could start a Patreon account and develop a complete China expansion; heck, they could call the project "Manifest Destiny" and take us all the way to the Pacific! I would help fund that--just like I would've bought this expansion, had it been what we were actually hoping for.
If the East is truly closed to us for all time, then I would share with you the sentiments of a great leader of antiquity:
"And when Alexander saw the breadth of his domain, he wept. For there were no more worlds to conquer..."