Well, there's plenty of ways to reduce one's wealth. Taxes, charitable donations (MacKenzie Scott), more ownership of the company by employees, and so on. But no, I don't think it is ethical for people to be billionaires.
What exactly are you objecting to when it comes to billionaires, though? The nominal wealth? The relative wealth? Whichever metric, where is the point where it goes into "immoral" territory and what is the justification for choosing that point over another?
I'm objecting to the idea, ethically, that a single person can own so much money. 1 billion is far beyond what anyone needs to have a comfortable life.
Whatever the line is, 1 billion is clearly on the other side of it. I'm not interested in sorites paradox-ing the question beyond that.
That's alright because most billionaires don't have that much money, they have wealth, which isn't really the same.
Also there's probably lots of billionaires here in Indonesia, where 1B IDR is only $67k USD. Do you object to Indonesian billionaires as well? Or does it only matter if you're a billionaire in USD? What if I'm a billionaire in GBP?
Do you really believe that everyone on earth should only be allowed to have exactly what is needed to "have a comfortable life" (which definition of comfortable we're following seems like another quandary that we'll ignore for the moment) and nothing more?
Yes, in general I think people shouldn't have more wealth than is needed to live a comfortable life, for some reasonable definition of comfortable. I'm not a communist -- I don't want everyone to have exactly the same level of wealth or anything like that. I think it's great that people are rewarded for pro-social activities (though capitalism has a very narrow definition of pro-social).
As I said before, I'm uninterested in sorites paradoxes.
I think the fundamental issue with your philosophy is that by focusing on people that "have too much" instead of primarily worrying about people having too little, you will end up in a situation where you actually have a harder time solving the latter because people don't bother to strive for the former.
Your viewpoint is a pretty standard neoliberal one. Personally I consider the existence of such massive inequality to in and of itself be a problem, separate from but related to the problem of poverty.
Which I must admit (to me) is a patently absurd viewpoint, and probably the entire source of our disagreement.
It is a moral issue when someone has great excess that could be redistributed to people who are not yet at the "comfortable life" standard (whatever has been agreed upon). However, if everyone is already at the comfortable life standard, what could possibly be your justification for saying that one guy having more than another is still immoral?
Earlier you said "I don't want everyone to have exactly the same level of wealth or anything like that", but honestly it sounds like you do. If your moral basis isn't "we need to fix poverty" but rather "delta bad", then your ideal must be for everyone to have the same, anything else is internally inconsistent.
And I will point out that despite the other half of your protestation ("I'm not a communist"), this is an unfortunate trait you share with most communist regimes of history – more interested in the "eat the rich" part than the "feed the poor" part.
It seems to be that statement needs to be justified with a little bit more than "because I say so".