I don't understand high finance. The Chinese are not buying so if the Fed stops buying US debts, who is?
Couple of things...
1) China (and others) may be NET sellers, but that doesn't mean they have stopped buying. Some portion of debt holdings (eventually) expire.
2) As the worlds reserve currency, USD, US government debt or hard dollars are REQUIRED for a nation to conduct business in the global marketplace. In many/most countries, a reserve is maintained for things like energy and other commodities (to name a couple), as well as other financial conversion and accounting reasons.
3) As long as confidence remains in the full faith and creditworthiness of the USA, as long as the USD is the worlds reserve currency, as long as (civilized) societies exist, there will be some degree of buying.
4) Some nations impose mandatory purchases of their own debt by banks, and other financial operations, and even certain financial vehicles. The USD does not have such policies, but imagine a 401k or IRA being required to hold a percentage of the account in US debt. It is not impossible, nor would it be a first. I'm just saying, not forecasting or speculating such will happen. Just that it could.