What prevents them from doing so now? They can already scrape public internet activity and create social credit profiles based on that.

Here I am posting a picture of Winnie The Pooh which I know Xi Jinping absolutely loves: https://ohmy.disney.com/wp-content/uploads/2016/01/Pooh.jpg and my "social credit" is presumably now at zero.

Thankfully I am not in China, never will be, so even if the Chinese social credit system hates me I can still take a train, get on a plane, etc.