My understanding is that many YouTubers use Patreon not only to supplement their income, but also as insurance against their videos getting demonetized over issues they may not have control over. Don't put all your eggs in one basket, as the saying goes.
The bet being made, I think, is that when Patreon is doing several times current revenue, Google will want the paying users and their credit card numbers and will be happy to pay for them by acquiring Patreon.
Even though Google could recreate the whole Patreon system, they'd be forced to start with a customer base of zero and risk pissing off Patreon's customers and creators by trying to get in between them.
Also, as others have pointed out, Patreon is not just Youtube. Google tried to clone a product and a network before, with Google+, so they know how difficult that can be.
Doesn't seem like Youtube was particularly good at copying established business models in the past (streaming, video on demand, etc.). But you knows, at least for the video-producing crowd (what fraction of Patreon users is that even?) raising money through Youtube donations might be an alternative.
Youtube is still secondary to Twitch when it comes to serious online gaming streams. I guess Youtube has a big "Lets play" audience, but that hasn't really eaten into Twitch's core audience.
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The thing about Patreon is that its multi-medium. I've seen good video games, webcomics, books, AND yes, Youtube Channels, which are funded through Patreon.
Unless people start selling their Art, Video Games, Books, Webcomics and so forth on Youtube, I think Patreon will be fine.
