I don't think subsonic stealth plane is that big of a threat to current day China. First of all, stealth is only going to get more and more useless as sensor technology and AI gets better and better.
Radars are progressively getting more and more powerful these days. So they detect smaller object from longer distance. Moreover, AI is getting more smarter, which means even small radar signatures can be identified as a stealth bomber with pattern recognition.
Then there is detection using IR and visible light itself. No amount of stealth technology is going to work against that, unless you have technology to make planes transparent. Due to AI and extreme high powered camera type detection systems, you will have ability to detect objects 100s of KMS away.
Moreover, China will have enough radar, planes and ships to essentially forward deploy and act as forward observers during wartime. So, any B-21 that comes close will be detect way before they come anywhere close to the mainland.
I am not even counting satellite detection since they could be destroyed in a full scale superpower war. But they are an option too.
I think all of these factors will make B-21 detection so much easier that there will be marginal difference between a B-21 detection range and a B-52.
If China can keep B-21 at a distance then it can only launch very large hypersonic missiles to cover the long distance. That will limit how many missiles it can carry in its internal bay. A B-21 that can only carry 1-2 standoff hypersonic missiles is not a big of a threat to China. If they try to carry smaller subsonic missiles, well those missiles are easily intercepted.
You are completely wrong about stealth. It does not become useless as sensors proliferate and networks improve. It simply becomes the new minimum baseline of survivability. The same systems which detect low-observable signatures will detect big open targets even farther away. The same algorithms which parse minimal signals from background noise will find loud obvious emitters even faster.
It's a spectrum, not a binary. Stealthy platforms are more survivable than non-stealthy ones. Everything that makes stealth less useful makes non-stealth useless. Today, stealth is a strength. Tomorrow, stealth is a requirement.