Would anyone besides those within China themselves use a completely closed and hidden model from China for their critical business needs?

If the outputs are independently verifiable at low cost, and the US models refuse to even try because somebody sneezed nearby and it sounded like "antigen" and not "achoo"... yeah sure. Whatever works to get the job done.

The hope is that AI will open up whole new sectors of economic activity. If you have to chose between exploring that space while potentially being exposed to Chinese tampering, versus just sitting on your hands and doing nothing... well then you take that risk.