I think if we jump instantly to a fully automated world, perhaps there's no problem, but in a normal transition from the actual world the issues are
- capital / limited goods
- allocation of labor
Things like land are a limited commodity. Concentration of ownership usually comes with rent extraction. Because you need to live somewhere, but available locations are finite (and supply can even be controlled, and is). This means that you can end up with a cartel scenario where increasingly normal peoples previously disposable income is tied up in an increasingly large % to those fixed costs of living. This is arguably well under way in some countries.
The second point is, the richest enjoy goods like personal yachts, private planes, luxury restaurants. If a lot of labor is increasingly allocated to producing highly inefficient luxuries, it's essentially wasted. You might have hundreds of businesses employing thousands of people or machines, but the productivity of their work is essentially all going into products consumed by a few people. I'd say a limited amount of this is a good thing, it's a normal part of the proliferation of choice enabled by capitalism. But taken too far you return to a historic economy where most peoples productive life is spent serving a tiny elite, while living in poor conditions themselves.
In our financial system, your income as an individual has to be significant enough to send a signal to produce the goods you want. If this ceases to be true for even a significant minority of people, you will find consumer choices and available quality decreasing for those people, which is a real problem. This is to a lesser extent happening in some segments of the economy.