It's a reliance on a (single) counter-party continuing to find the deal attractive. If, for whatever reason, they stop, the cost of debt will explode and bad things will happen. As you mention, it's a policy. Policies are subject to change. The Chinese economy is maturing into consuming more of its own output, and Chinese people are growing wealthier and more interested in buying foreign goods - both are long term trends that makes that policy less attractive.

The reason it's hard to sell that the existence of cheap debt as a signal to borrow and spend, is that the US isn't paying down its loans, it's rolling them over into new debt and assuming that the new debt is not significantly more expensive than the current.

That assumption need to hold several decades in the future. If it fails, things get real ugly.

EDIT: By "bad things" and "real ugly" I mean significant, sudden inflation which voters are unlikely to reward.