The hollowing out of the middle class, wealth distribution producing more unequal societies where few people amass wealth and more of the (previously just working class but now more and more of the middle class) are pushed out a lower rung of the income/wealth bracket and ultimately completely displaced.
Fundamentally in a healthy economy the money is the blood that circulates around. If you create an economy where most of the wealth is held up by few rich individuals who then use the wealth to buy more assets and produce more wealth it means that other less well off people will get displaced in the society and that means that they will also not be able to afford to participate in the society to buy goods and services simply because they have no financial means to do so.
And to those who immediately go "but the economy isn't zero sum game", yes that's correct. But when the real economy is growing at 1-2% rate and the musks and bezos and other wealthy individuals are growing their wealth at +20% per year that means it effectively is a zero sum game and some people's slice of the pie is ever growing at the cost of everyone else.
But then people say "but the rich people invest". Yes but how much of that is actually active investment in new businesses and how much of that is just investing in asset portfolios, property and other financial instruments?
Also to grow more businesses and start new businesses you need an individual who has financial means to take risk, has an idea, stomach for risk and determination and drive to start business. Many people do not have that.
For the economy to grow new businesses which one is better? 1 person with $1m and 9 with $0 or 10 ppl with 100k each? What are the chances and probabilities in each scenario that the person who has the qualities to start a new business is capable of doing so?
If you want to see the future (especially relevant for US and UK since they're furthest along the way) just look at the economy of Nigeria or India for example. Few rich people, tiny middle class and large swathes of people in poverty.