1. Apple adds easy support for non-Apple app stores and side-loading to iOS to comply with the recent EU regulations that require opening things up in 2024, and then people can download TikTok from outside the US and install it?
2. TikTok users switch to using the TikTok website instead of the app?
It looks like the main thing the app gives you that the website can't is a convenient way to film short video and edit it and add music all on your phone and then post that. Surely someone could write a social network agnostic app just for filming, editing, and adding music that can upload that short video to any of your social media accounts (TikTok, YouTube shorts, and whatever other ones allow video). The destinations could be entirely user configurable and support any social network that provides a halfway decent upload API.
What's the US going to do? Try to make a US equivalent of China's Great Firewall? I don't think that would work here, because our free speech laws make it too easy to circulate circumvention information.
If I was a company that does mobile apps I'd be seriously looking right now into making that general short video maker/uploader app. If the US does successfully cut off TikTok all those users aren't going to just stop wanting to post and read the kind of things they are now doing there. They are going to try to move to other platforms. Done right, maybe my app would be something they use as part of that.