That would get you banned from 95% of Reddit subs. (Kidding, mostly..)
Reddit is ruled by a cabal of "leftists", but of the "progessive" strain, rather than the "tankie" strain. Criticism of China and communism is fine almost everywhere.
I honestly see more CCP apologists on here than I usually see on reddit (though I don't hang around the places where I'd assume they usually comment I guess, with the exception of a certain popular gacha game subreddit). For example: try mentioning the Uyghur genocide here, you'll be surprised how many people come out of the woodwork to muddle the conversation.
As you say, it probably does depend on where you hang out. I've never detected sino-simping on any non-political subs i read, but because, for my sins, i read a few mildly unorthodox left-wing subs, and they are crawling with CCP apologists, tankies, vulgar anti-imperialists, and so on and so on.
Except China isn't exactly communist. They're a "socialist market economy." Yes, the government exerts a lot of control and owns a great deal of industry, but there's also a significant private sector. It's basically state capitalism, closer to fascism with a blending of state and corporate interests.
Communism is not just another word for authoritarian. Totalitarian regimes come in a variety of economic flavors.
Why do you think it's fundamentally fragile? The soviet union lasted for about 70 years. China has had a unique party for around 70 years (and no signs of ot changing anytime soon). What other types of government are less fragile? I think democracy is more fragile for sure.
The US has lasted well over 200 years.
Well unless you skip over that whole Civil War thing where the states weren’t exactly “united”
The Union remained a democracy throughout that war.
Overcenralization in an environment of increasing disruptive dynamics is a path to eventual catastrophe when survival boils down to minimizing latency to action.