Since the wealth of the rich accounts for 80% of the total social wealth, isn't encouraging the rich to spend enough?

To answer this, we first need to understand the three insurmountable hurdles of capitalism.
The first hurdle: the Matthew Effect
This is a common flaw in all private ownership systems, not unique to capitalism. In the early stages of private ownership, resources are abundant, the starting line is roughly equal, the more you work, the more you earn, and everyone has a bright future. But by the mid-stage, large capital begins to show its size advantage. Coca-Cola relies on scale to lower costs, and new small factories simply can't compete. Large companies can also clear the market by losing money, such as early Meituan and Didi, quickly establishing monopolies.
In the later stages, resources become scarce, and the big fish start to look ugly. Feudal dynasties are examples: after wars, populations decrease, and landlords and commoners each have their own land to farm. But landlords use various means to seize land, such as low prices during natural disasters or colluding with officials to fraudulently buy out land. Eventually, peasants own less and less land, landlords form solid power, and they oppose imperial authority while squeezing the people. During major disasters, if people can't survive, they rebel; or if the emperor wants to centralize power and local warlords rebel. Old dynasties collapse, and the people go through dark ages, waiting for the next dynasty. This is the historical cycle law mentioned by Huang Peiyan. As long as private ownership doesn't change, the Matthew Effect will always exist. Capitalism needs to be overthrown not because of moral issues among capitalists, but because of systemic problems. Good capitalists can more easily change their mindset, while bad ones are more stubborn, but the key point is that "you" versus "me" is not the most important.
The second hurdle: capitalism is not fundamentally aimed at production
Wealth only grows in two ways: creation and transfer. Creation means producing, making the cake bigger. Transfer means taking, whether rational or violent, always a zero-sum game.
The core purpose of capitalism is capital appreciation, which is the Matthew Effect—methods don't matter. Early capitalists were eager to produce because social wealth was low; only by developing production could they accumulate wealth, the first pot of gold. But once capital accumulates to a certain extent, they start to think crookedly because production is slow and exhausting, leading to the birth of finance.
The most common example is bank lending, earning interest on loans. When capital is small, interest isn't very powerful; when capital is large, it can achieve financial freedom. Several hundred million lying around can generate tens of thousands of dollars daily without engaging in any production, contributing no direct increase to social wealth. Although bank interest indirectly creates social wealth, other forms may not be reasonable. For example, owning ten apartments in Shanghai and collecting thousands of yuan in rent monthly, a PhD student’s monthly salary is about the same—landlord is just a position that can be held by a dog.
The power of capital becomes evident: doing nothing daily, yet wealth rivals that of top talents. But behind this peaceful life are countless workers sweating blood. Capital is an abyss that makes heroes bow. Jack Ma transformed from a spiritual leader who made business easy for everyone into a benefactor, and industrialists, after growing big, all invest in real estate. If the government didn't regulate finance, there might be a bunch of private banks today. Having tasted financial freedom, it's hard to go back to the hardships of building from scratch. The government has always wanted money to flow back into manufacturing, but with little effect. Besides real estate and stocks, there's Bitcoin—anyway, labor is impossible to truly labor.
The third hurdle: capitalism will ultimately stifle consumption power
This is the question the original poster asked. Why doesn't encouraging the rich to spend work?
A person's accumulated wealth can be infinite, but the amount they can consume is limited. If wealth can't circulate, it becomes meaningless. Jack Ma eats Australian lobster once a day—365 meals a year; he buys a Mercedes once a day—365 cars a year; he buys Armani once a day—365 pieces a year. He earns billions a year, but his normal consumption might only be a few hundred million, and the rest of the money can't enter the market for circulation, making it impossible to create wealth. Money only exists as money if it circulates; stored in a warehouse, it's just paper.
Early capitalism stimulated consumption, but after the Matthew Effect took hold, big capital began to suppress consumption power. The more money Jack Ma earns and the faster he earns it, the less wealth is left for ordinary people to circulate.
Can Jack Ma overspend on consumption? Eat one and throw one away? That's far from enough to activate the market; the media might even criticize him for extravagance. Buying 100,000 Australian lobsters daily? It would indeed activate the market, but one person can't finish them all, and throwing them away is like the milk that spoiled in the US over 33 years—done, and probably ready to be thrown on the street. Not sharing all of it with Chinese people? That’s wealth redistribution. So what’s the point of earning so much if you’re just going to die with it? It’s better to earn less from the start and share more.
From a capital perspective, every capitalist is just a puppet of capital. Once in the capital game, there’s only one task: keep making money to expand capital. Because in an environment of the Matthew Effect, slow growth will be eaten by faster-growing big capital. Overspending contradicts the nature of capital; doing so will cause capital to abandon him and be replaced by other capitalists.
This is why capitalism doesn't work.
We don't know if socialism can work, but following the capitalist path will ultimately face these three hurdles. Once you reach the end, it results in economic crises or world wars, system collapse, and mass casualties clearing space for survival, allowing future generations to start anew.