Nothing is forever. The current regime in China won't be forever either. The question is how long.

After Augustus Rome still had a couple of hundred years of expansion where a nationalist could argue, “hey look at that! Things are still going good.” But they couldn’t outrun the rot in the system forever.

After Xi will China get another strongman? I think this is the key. If there are a series of total dictators things will decay. Whatever justifications a dictator uses for their rules, ultimately they are going to try to implement policies that keep themselves on top first.