I doubt anyone at this point is holding more than 100-200k XMR, in which case the price would have to rise to $5-10k per coin to make them a paper billionaire.
Nobody here wants to predict that kind of price but all I will say is that when I was buying BTC at $225, there was no way I would have said we would see $4747.54 (current price) per BTC... I couldn't even imagine it. And I would have been mocked for calling out a price like that with no evidence. Now, we see the predictions of BTC at $10k-15k-100k even $500k a piece and if we believe that Monero will take 3rd, 4th or 5th in marketcap, what price would it need to be?
It's an interesting question where it will be, in even a relatively short time period i.e. 2-5 years.