Ordinary workers would be much more motivated for example. And it would definitely produce fairer income distribution. Why not give it a go? Why should holders of capital take income from doing nothing? Why is that fairer that a system where the workers get the full benefit of their own labour?
All of these questions assume that humans are the same. We're not! We are all different! Some lead, some follow. There is nothing wrong with following, it just pays less. There is also a lot less responsibility, and thus lower stress. Some melt under stress, they can't lead. Others are at their best under stress, thus they are paid to perform under the conditions.
What motivates one person does not motivate another. Some are motivated by money, so much so that they work like a dog to get it. Some of these are also intelligent beyond the rest of us. Combine the two and you have an almost unstoppable force in accumulating money, building businesses, and leading other humans.
Others like money, but are willing to work just enough to sustain their needs, and spend the rest of their time elsewhere. Again, nothing wrong with that, I would rather hike and bike most weekends rather than work extra, and I would say probably a big percentage of the population falls in line with that kind of thinking.
You said "why not give it a go?" Isn't that what the Soviet Union did? How did that work out? China recognized they were going down the same path, so they reformed in a big hurry.
Again, you can't restrict capital without hurting everyone, not just the "rich." There always have been winners and losers. There always will be. The best way alleviate these issues is to have equality of opportunity, allowing people to work/build businesses/invest to their own level of satisfaction without punishing them with the heavy hand of government.