On one hand, speaking for the US, far too many struggle because their cost and risk exposure far exceeds their income. For too many, it exceeds their income potential.
Billionaires are a product of that. Too much went away from labor.
That trends immoral. And it is currently legal. Corruption itself is basically legal. Maybe it should not be, or... better:
Say the US was a more robust social democracy. The vast majority of people have income appropriate to their cost and risk exposure, do not struggle, live modest, reasonable, fulfilling lives.
Would this question come up outside smallish circles, rather than the more general discussion happening today?
I know I would not care. I care very much in the current scenario. Too many are in real trouble, struggling, working more than is necessary.
I have arrived here:
Do the people right. Beyond that, make as much as you can.
It is harder to make billions in that scenario, but the net happiness in the world is far greater.
Say someone makes a few billion, or even just one and change. What is the meaningful difference in happiness between that and tens or hundreds of billions?
Agency is about all I can think of, and really that is a different question:
Should individuals command nation sized resources?
I would argue no.
Let's say someone runs a shoe store. When someone wants their money more than they want new shoes, they walk past the store, unmolested. But when they want new shoes more than they want money, they go into the store. They willingly give up their money to get something they want more than money. That is, the store makes all their customers better off - not better off in terms of money, but in terms of "money plus stuff". The customer gave up something they wanted less in order to get something they wanted more - they're better off in their own terms. If the store owner can make all the customers better off and get rich doing it? Then I agree with Deng Xiaoping: "To get rich is glorious."
To get rich by exploitation, by theft, or by deceit is shameful (that is, the person doing it should feel shame, whether or not they do, and whether or not the society actively shames them). It is shameful in China and in America; it was shameful in Deng's day, and today, and it will be shameful forever.
So: Being a billionaire is not inherently immoral, by itself. But it depends on how you made the billions.
One could even argue Bill Gates' comfort of not cooking, shopping or cleaning for himself is good, given he uses is power/wealth for good more efficiently if he doesn't do chores. (putting has past aside for arguments sake)
So I'd say it usually is immoral, unless you use almost all your wealth to do good.* *
It gets really interesting if we ask if being rich is immoral. I'm not rich in my country, but I'm rich compared to people in other countries. Does this, especially with our colonialist past (and present), make me immoral? I think it does and yet it would help little if I lived a more frugal life either.
I'm living a compromise and it's biased towards my well being. I'm living very moderate, what I don't need I save for later, not much travel either. And yet I own good speakers, purely for my pleasure, financed by my privilege of being a white male in the "western world".
I guess "being a billionaire" in itself is not immoral - how one gets there, and what one does to stay there is what defines how immoral the person is.
If you use your wealth and power for good it can be a great thing for you and for society.
Providing good jobs and benefits.
Building more companies that treat employees well.
Donating to charities.
But yeah, as a non-billionaire, I'd say that when things get to that scale, you become less moral compared to the mass. People's life becomes just numbers.
Would that food have been produced if you didn't have the motivation to hoard it?