I don't know if this will have a huge effect. First, China tends to try and keep people employed even if they're economically unproductive, in places like state enterprises. That means there's a lot of slack, a lot of jobs that could get cut without causing any adverse effects. Second, the trend towards roboticization of production lines is accelerating, is wiping out whole categories of jobs, and the robots themselves are often 'Made In China'. If you go on AliExpress and look at anything from pneumatics, to hydraulics, to electronics, to complete machines for doing stuff like packaging, you'll see it's all available domestically at a bargain basement price. All of these things increase worker productivity. If Chinese factories hit the level of automation you get in US factories, then it won't matter that the workforce is tiny and it's supporting a ton of old people.

Way to many people work on filler jobs that will go away and automation will also solve a lot of the problems. China has about 22% people working in agriculture, that will go down a lot over next two decades.

Would that mean the population then will be much wealthier (as happened after the Black Plague)?

Definitely. Unprecedented in human history. Wide-ranging social, cultural and economic impacts.