He's absolutely right about risk and reward, but he's also not seeing the whole picture.

Ideally inequality shouldn't matter. Poverty should matter, but why should it matter if someone has billions of bucks?

But our world is not "ideal."

The problem with inequality is what happens when you have enormous excess savings at the top. It has to go somewhere, since sitting in cash is almost never the best thing to do with a lot of money. A deflationary currency would let it sit in cash, but that would also have a negative effect on all forms of investment since it makes cash a "sure thing."

Ideally it should go into productive investments: new companies, investments in expansion of existing ones, R&D, etc. Some of it certainly does, and this is what's driving the present boom in seed funding, VC, and other beneficial things.

But again, our world is not ideal. There is not an unlimited supply of good investments. If the amount of money saved exceeds the supply of good investments, only two things can happen: (1) it chases bad investments, and (2) it inflates asset bubbles.

(1) results in investors losing most or all of their investments, so investors are going to shy away from that. Why would you invest money in something with no chance of success? So as the investment market gets saturated, there's going to be a limit to the extent to which investors are willing to lower their standards. They might take somewhat more risk, but they're not going to invest in what they see as obvious losers.

That leaves (2), and that's where things get ugly. In short: your surplus savings is why I can't afford a house. Your savings are hurting me. They shouldn't, but they are.

Surplus savings get dumped into idle assets that seem as if they're likely to hold value: real estate, commodities, etc. This inflates the cost of those assets, effectively imposing something like a tax on those who actually need and use those assets. Now people have to take out bigger mortgages, or even if they're cash buyers they have to lock up more and more of their personal wealth in their home in order to afford the artificially inflated prices demanded by the market. They have to work harder and longer for less real payoff in order to prop up the savings of the super-rich.

As this continues, there's less and less money available in lower echelons of society for other things. This in turn results in less profitable investments. Why invest in building new businesses when there are no customers for those businesses? It's a vicious cycle: higher rents and asset prices lead to less disposable income which leads to less opportunity for new businesses which leads to a worse investment market which leads to more money being parked in rentier assets.

Eventually you end up with a true feudal society: huge amounts of money at the top and everyone else as renters. Only the nobility can afford to actually "own" anything, as any own-able durable asset has become a bank account for parking surplus savings.

Edit:

Returning to (1) and (2) above -- chasing bad investments vs. inflating asset bubbles -- what we've seen has been a progression from (1) to (2). As inequality exploded in the 90s, it first inflated a VC and stock market bubble. Then this bubble popped and all that money ran into real estate and other rentier assets, inflating these to sometimes ridiculous degrees. There was a partial pop of the real estate bubble but it was rapidly backstopped by the state, since at this point the choice was between a perpetually inflated asset bubble and a great depression.

The current reality is "in-deflation": inflation in everything you need (a.k.a. rentier assets), deflation in everything else (a.k.a. produced goods). It's possible that we'd be better off now if they'd let it all burn in 2008, essentially destroying all this surplus paper wealth. But it's hard to say.

Edit #2:

I wonder if these problems would exist if humans lived longer? Keynes correctly describe what I outlined above as "savings in excess of planned investment," and life span effectively limits our ability to plan investments.

There are tons of great investments that could easily soak up all of today's surplus savings, like terraforming Mars or sending interstellar probes in search of extraterrestrial trading partners for example. But who's going to invest in something that wouldn't see a return for 10,000 years?

That's what makes Elon and others like him stand out among their rich cohorts: they invest like immortals.