‘Instability’ and ‘unstable’ are words with interpretable meanings.
Capitalist markets are dynamically unstable. They are supposed to be. That’s what makes them work.
A human being walking is dynamically unstable. That’s what enables him / her to walk forwards efficiently.
Water in a bath is stable. Until you pull the plug out nothing happens.
Water in a river flows.
Capitalism is the most efficient economic system for generating wealth precisely because it creates dynamic instabilities and resolves them promptly. Capitalism highlights needs and shortfalls though the pricing mechanism, and this encourages suppliers to address these needs and bottlenecks promptly. In this way capitalism fixes bottlenecks to supply / production, and encourages innovation, far, far, far more efficiently than any other economic system. Economic decision making is decentralised and efficient based on market prices.
Which is why it is important to regulate markets to ensure they are free and efficient.
In short - capitalist markets are in a constant state of dynamic instability and flux which capitalism addresses.
Stability is reached when the price of everything hasn’t changed in years. That basically means the economy is dead and nothing is moving - See The Great Depression.
A friend of mine asked if anyone had yet figured out how the global economic system works. I pointed out that as soon as three major parties - say Europe, the USA, and China, all agreed on how the global economy works, that necessarily would change how it works. As soon as anyone thinks they know how the market will respond, they can take positions to short the market, thus changing how the market works. That’s capitalism in action. It’s dynamic, not stable.
A capitalist market is efficient because it is a constant evolutionary battle - survival of the fittest. It’s a jungle. Ecologies are sometimes stable, but commonly not. 99% of all species that have ever lived are now extinct, but the Earth is still teaming with efficient evolved life.
Capitalism works the same way. It constantly forces all suppliers to strive for greater efficiency.
To understand a capitalist economy you need to understand Game Theory. There are lots of players all competing with different strengths and goals. Yes - it’s dynamic, and usually unstable. That’s what makes it interesting and effective.
People standing around are stable, but not going anywhere. When they start to move they are unstable, but that’s when things happen and we get places. Yes, occasionally there are collisions and somebody falls over. But that doesn’t mean it’s better if nobody ever moves.
So - yes. Capitalism causes some people to earn more than others. Capitalism causes price fluctuations. Capitalism sends some businesses to the wall, and people lose their jobs. Capitalism encourages the introduction of new technologies, and more people lose their jobs. And new companies start up and new jobs are created. Constant struggle. Constant change. A constant battle to innovate, to solve problems, to be more efficient.
And that is why capitalism is far and away the most efficient wealth creation system Mankind has invented. It forces players to make constant efforts to improve.
Hope these thoughts help.