That's a very poor way of comparing wealth. For example, the difference between $1,000 and $1,000,000,000 is also about a billion, but $1,000 is 0.0001% of $1,000,000,000 while $1,000,000 is %0.1

Buying power is not linear in today's world, either; at best, it's logarithmic. Because the law of diminishing returns is a thing, you can sink as much labor (and value) into goods and services as you want, and the quality will eventually reach an asymptote: it will approach perfection, but never get there. This means that in general, the difference between what millionaires and billionaires can afford is measured in quantity and not quality: billionaires can afford 100 yachts or one megayacht, but millionaires can still afford one of those near perfect yachts. Billionaires can afford many mansions, millionaires can still afford at least one. The millionaires have more in common with the billionaires than they do the average incomers ($60,000/yr in the USA). I make 3/4 of the average income, but I'd be lucky to afford a used jetski, much less anything resembling a yacht.

Put another way, in a society where everyone's a millionaire, the richest possible billionaire is 1,000,000 times more wealthy than the poorest possible person, and the average billionaire is only 1,000 times more wealthy than the average millionaire. Whereas right now, they are infinitely more wealthy than the poorest person, and the poorest possible billionaire is still 17,000 times more wealthy than the average person in the US.

So what I was originally getting at is that the mere fact of billionaires existing can only approach some semblance of morality in a society where their wealth doesn't put them on an entirely different plane of existence from the poorest people.