I'm more or less in the same situation as you, although relatively early in my career. I would urge you to try and look a little broader at what situations people even just below "middle class" are facing nowadays.

From a personal finance perspective, I'm aligned with you. I shovel my money into my 401k and index funds, and still have plenty left over to survive and buy things I want and need. By doing so, I can participate just a tiny bit on the finance side of capital and take in a bit of the upside.

>I understand those below the poverty line may not have the luxury of opening a IRA, stock account, etc. or immediately starting their own online business.

It's not just people below the poverty line. The vast majority of Americans, especially younger ones, are not making enough to substantially invest or feel secure financially. If someone does start their own business (an incredibly difficult venture), it will be at their own risk, with much to lose. Dabbling in side projects is a luxury, and I don't think it's reasonable to expect people who are more overworked and less financially stable than you or I to realistically use that as a way to lift themselves up. It's also not really a solution that scales, in my opinion. You can't expect everyone to start their own side hustle. Most businesses fail. Some lose money for years until eventually breaking even. Obviously, more opportunity is good, and I want to see people succeeding in starting their own businesses and innovating, but isn't having financial and social safety the biggest factor in encouraging that?

>Make this article relevant to a middle-class person on track to retire in their 60s, who tinkers on weekends with a few startup ideas, and generally thinks their kids will be better off if they continue to have the same access to education, business and investing. Why should that person be personally concerned about income inequality?

I'll take a stab at this from a couple angles. First off, you either care or you don't. Like you said, you're doing fine. I'm doing fine. I don't think anyone really owes you an explanation to make you care, because you don't have to. But I have friends who graduated at the same time as me who aren't doing great. They're working way harder at whatever blue collar job they could find than I ever did, for less money. Mental health isn't great. I'm not particularly different from them, other than I happened to like computer science as a kid, and my brain is sufficiently configured to churn out some code for an employer for 8 hours a day. I don't think I could look them in the eye and suggest that they just invest more, or consider starting their own business, or try online learning, when they're already down. I never did any of that.

The other side of it is that the middle class is still significantly more aligned politically and financially in their interests with the rest of the population than the 1%. I'm making a lot of money, but one bad trip to the hospital would still bankrupt me. I still worry about rent. How about everyone else who is worse off than me? The truly wealthy are playing a fundamentally different game than the rest of us, however you feel about that. One interesting aspect of the recent push for more progressive policies is that much of it is being driven by middle class white collar workers, and not blue collar. I see it as a sign of the increasing income/wealth inequality trending over the last several decades. The middle class has typically been 'placated' by being able to live a decent living, but even they're feeling the squeeze now.

I hope that was somewhat coherent and didn't come off as too combative. I really just want to convey my personal experience and how it's different from yours.