Good question, and it's a risk I took when first testing out the systems. But against initial expectation, I did not lose patrons while the reward systems were going on. Quite opposite, it brought more attention to my patreon's existence and I got MORE PATRONS.
My theory as to why this is the case:
People WANT to support their favorite artists, but not everyone can. For some, it's a matter of not having the money. For some, coins take too much time. Tipping reward systems allow you to engage further with your audience, thank those who support you with coins, and it generally makes people view you in a more down-to-earth light.
Some of my tippers have moved over to patreon and pledged more than any reward tier available simply because they realize the patreon exists and they want to support the content they enjoy. Some people see the tip rewards, go "that's awesome, but I don't have time to watch ads because I have a job." Then they see the patreon, and realize they can get those awesome things without having to spend time earning the coins.
Basically, patreon appeals to people who are in their twenties and above and have a stable income.
The tipping reward system appeals more to students and younger parts of your audience, or older ones who have low income because they aren't offered enough work hours at their job.
Both of these groups want your stuff. Why would you limit yourself to only reaching out to one of them, when you can reach out to both and create larger satisfaction in your audience at large AND earn better revenue so you can spend more time creating the comics they love?