If China stops accumulating Tbills, that would be a direct result from less exports to the US. If they stop buying them on rotation... letting them expire and doing nothing... that would mean they would have too many US dollars in cash, not earning any returns. So they would have to buy other USD denominated assets, which will be difficult for them since Trump is pushing against foreign investments in the USA...
Or they would sell US dollars for something else. That would also go against their wishes of a low valued Renminbi. They are already looking at lowering the RMB as an options in the tit-for-tat in this trade war.
So to me, it seems China isn't just going to stop buying Tbills... they would hurt themselves more than the US.
The US on the other hand wants a lower USD vs the RMB... but they also like the low interest rates they pay on debt. The last is a consequence of the USD being the reserve currency whereby everyone wants to hold USD and invest in things like the Tbills.... and it being used in the major commodity trade.
Unless the US actively pursues stepping away from the USD being the worlds reserve currency... shit isn't going to work out exactly like Trumps wants. And I doubt the US wants to stop with it, since their debt would become a bigger problem.
So... basically the US is still stuck in having to be competitive on the global market through innovations in design and efficiency.... not through competing with a same product same price level... Nothing wrong with that, but it seems Trump is still missing that point.