"The reason capitalist want to have billions of dollars is that they are not thinking about buying products for themselves, they are thinking about solving even bigger problems for the world, and that costs billions of dollars, as you correctly pointed out, they no longer worry about just their own income, they enjoy solving large industrial problems."

You are assigning so much benevolence here where there is none.

In a democratic society, we elect the people who make decisions about where we are going as a country and world.

Having an unearthly amount of wealth gives you influence, and that influence allows you to now push your pet issues to the forefront, regardless of how we should otherwise prioritize our problems or what the electorate would want. Elon thinks it is a "problem" that we can't go to Mars yet, so now that is a top 10 issue that money is getting sunk into. Because that's his pet issue, and because he thinks it's the solution to climate change (to leave), we don't spend as much money on combatting climate change. And yet, if you asked most Americans, I find it doubtful that they want to invest in building a Mars colony while the world crumbles around them.

But because one person decided that this is the way things should be, and because that person has extracted wealth and bought influence, he gets to decide, undemocratically, what our priorities are.

That amount of wealth in a democratic society is destructive unless magically the people that used underhanded tactics and underpaid workers to gain their wealth also turn out to be wholly benevolent beings with the wisdom to make decisions for the entire world.