The basis for why this is at all acceptable as a system to organize society is that competition, government restrictions, etc, whittle down private power to somewhat tolerable levels. There must surely be a better system which we haven't thought of yet though.
Many people, especially Americans seem to confuse capitalism with 'trade', 'competition', 'markets', etc, whereas I mean it as the private ownership of production, the profit from other people's work, etc. That seems fundamentally immoral in some ways from the very start.
The question then is how should we allocate resources (capital), how should we incentivize risk, innovation, hard work, etc, What kinds of ownership are moral, etc.
I suspect we'll be stuck with capitalism in some form for a while though - like democracy, it's the worst system, except for all the others.
The basis of capitalism is competition. The theory depends on no single company having the power to move the whole market, but this theory is getting decimated by the empirical reality of consolidation and monopolization.