If Android Goes Closed Source, what happens to Alternative Android-Based OSes, such as Graphene OS?

Also it's not going closed source it just doing closed development with pushing the code over the wall at release time.

"if"
No more input until it's already released?
- Yep i think they are looking to reduce down stream of releasing features before they do. It's shitty but it's not going closed source. They would have to rewrite the whole thing to go close source. They could do that but the version we have is ours they can't take it away.

They wouldn't get updates from base, anymore
That's not what's happening

What happens? (If, because again, that's not what has happened, so far.)

GrapheneOS would continue, but probably not manage compatibility with the next Pixel phone.
Various forks of Android Open Source get created, with mixed success. Existing phones will keep working, new phones with proprietary servers drivers are much less likely to work.
We create something like ndiswrapper to help us extract weird new phone hardware drivers and make them work with open operating systems.
The fully open phone hardware projects suddenly get a bunch of new customers.

The last point is the only thing I'm confident is preventing Google from taking their fork of AOSP fully closed. (Yes, I know it wouldn't be legal. But I'm not sure their lawyers think they would be held accountable. Most people don't understand the existing laws Google already breaks daily since purchasing YouTube.)

Please read the article.

I think it would still be a good question to ponder, even though that isn't what has happened yet.

What IF Google decided future versions were closed source? How would that not affect our open source alternatives?

Well, you get an iOS. So, Google shooting themselves in the foot.
Google can decide all they want, but they can't close source Android due to the GPL.
- https://source.android.com/docs/setup/contribute/licenses says most of the Android userspace is Apache 2 licensed. While they can't close source the Android branch of the kernel, they could close-source new userspace code and it would probably diverge from the last open source release quite quickly.
  
  Realistically, that would probably be sufficent to make Android functionally closed-source, even if the GPL bits were still available.

Nothing

Pretty much nothing, because 99% of what everybody uses is proprietary blobs on top of Android anyway. The Andriod open source is absolute minimum barebones, with MS Paint like UI and basically no UX.