U.S. Unveils B-21 Raider, the Stealth Bomber Designed to Deter China
wsj.com
wsj.com
That might deter something if it was plausible that the US would declare war with China by using the B-21 to bomb Chinese targets. However it is highly unlikely that the US would do that unless attacked first, which China is equally unlikely to do.
This program rather sounds like an expensive way to keep the industry afloat and the know-how at the cutting edge.
If the B-21 sees action the most probable is to carry out air strikes on groups like ISIS or the Talibans (which, apart from Kosovo/Serbia, is what the B-2 has been used), and just because they will be available so might as well be used.
Today they don't want to destroy a big customer and part of their supply chain, but that price could be deemed low enough in all sorts of circumstances. The very high likelihood they'd get destroyed in return by our nuclear triad including manned bombers make the price including to the leaders much, much higher.
That's already the case so you've answered your own question.
Even in a conventional war neither side wants to get into WWIII.
A war between US and China is not winnable by either side and so, quite obviously, neither side will start one.