We will be more like China
We won't be like China. We'll be more insulated and divorced from Chinese media, culture, and conversation.
That's the stated end goal. Bringing up sharp walls between nationalities in order to control the flow of information between people.
If we keep TikTok open, we risk exposing American young people to Chinese norms, ideals, and social standards. We might even be exposed directly to Chinese mass media (ie, propaganda).
This is a real security risk, as it raises a possibility that younger Americans won't accept American mass media at face value.
Even the people pushing this law acknowledge that the threat is purely theoretical, no one is advancing any serious argument that data has been or will be shared or leaked or anything of the sort. It's just xenophobia, if they wanted to protect American citizens' data they would do something about the way they allow tech companies to just take your data and turn around and sell it to whoever wants it.
Bonus points because US government officials can punt a great bargain basement blowout sale to former US officials by forcing the sale of a company using said xenophobia.
