There are a fair few who've done it in the Youtube space (the infamous Pewdiepie for example), but no, I don't think I know of any Patreon millionaires yet either.
But how sustainable is it? Patreon is rather simple: Users directly pay the artists, however much they want (and Patreon presumably takes some fee).
Youtube makes artists depend on Youtube's subscription and/or advertisement viability, which are largely outside of the control of both artists and consumers. Consumers cannot control where their money ultimately ends up, artists have little planning security.
This is important. Typically one would have income from: youtube ads, twitch payout, sponsored content and patreon as a bare minimum. Usually you can add sponsored stuff on instagram/fb/wherever, possibly selling merch. Not to mention the stuff you get without being explicitly asked to endorse it.
Google says there are currently 2.9 billion internet users, if 1% of them pay you 1 dollar you'd gross 29.1 million dollars, 0.1%, moving the decimals even at just 0.001% could gross you a cool 29 grand. That's really not a bad deal.
This is actually why I like Patreon... the model seems to align more with sustainability.