This argument is flawed. The reality is very different, as observed in other countries with demographic decline.
As people get older, they spend less even though they have the money. That is because they don't need to spend on the many things that are required by families. Older people almost always spend less than younger people. In addition a large proportion of older people don't need to work because they have money saved. Or they can't work because of health issues..
This starts a vicious cycle, low spending means low demand, which requires lower production, which means less money in the pockets of people, which means lower spending.
China does not have a problem now, as it's population of older people is still low. However, 25 years from now, things will start getting bad.
The first thing to solving a complicated problem like demographic decline is to acknowledge that is exists and then think of solutions.
I talked mainly about working-age people (in the context of economic productivity), not the already people in retirement you are talking about.
Working-age people - as they move toward higher value-added industries, they will certainly have higher incomes.
(Those industries have fewer workers available globally, and higher profit margins due to lower market saturation allowing for higher pricing, hence corporations are forced and able to pay more their workers, due to the competition between them and due to the labor market supply/demand dynamics).
Hence, they will be 100% more than today (that's how human psychology and economics work, humans are natural spenders, especially when you give them more money). So the argument that quality improvements in population won't drive consumption is invalid.
They will not spend everything they earn, of course, as people in China are already very frugal and have the highest personal savings on Earth.
However, overall domestic consumption will still rise (btw domestic consumption in China is way less important economically than in the West, but still important a bit nonetheless). As for the older age pyramid in China at that time, that's totally another topic.
That's the topic regarding the pension system in China, etc. That could be solved by increasing the retirement age, or simply by the government having more income from taxes and other sources thanks to overwhelming increased productivity from the working population getting more qualified and paid in those 20-30 years.
Not many countries in the world today have a pension fund that is self-sufficient and can pay retirements on its own, governments often assist with it from their budgets, and that's nothing strange.
Also if we talk about health conditions, China has a way better health system than in the US for example, lower metabolic syndrome rates, higher lifespans, and probably overall healthspans too, so the retirement age could easily be raised.
However, China is a socialist country, so I don't think that they will even raise the retirement age, they will probably pay it from its budget if the pension funds stop being self-sufficient (I think they maybe already aren't, but we see no problems in debt levels).