> economic inequality per se is not bad.
A lot of research shows that high society-wide economic inequality causes many bad effects, economically, politically and socially. I believe that is the basis of many concerns. According to that research, high inequality is per se bad (I don't know enough to define "high", but many others have).
> [inequality due to startup founder success is good]
Generally I agree, but:
1) The question isn't whether all inequality is bad but whether current levels are bad. Nobody argues for perfect equality.
2) It overlooks the problem that many are denied the opportunity of such success because of their race, gender, social networks, or economic class,[1] which deny them access to the important resources such as education, financing, contacts, or jobs.
That's bad for the people who otherwise would succeed; it's bad for our society socially and politically to have people who are and feel excluded; and it's very bad for our economy to not utilize the >70% [2] of human resources that aren't middle-class or wealthier white males.
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[1] AFAIK, plenty of research shows that education depends significatly on the economic class of your family
[2] Around 33% of the U.S. population is white male and some of them are poor; therefore 70% seems a safe estimate.