Apple

I'm curious if you can point me to any specific instances of third party app stores being problematic for Android, perhaps I missed something.

In my experience, most users either don't know about the third party app stores or don't use them. Android has a setting to completely disable downloads from outside sources. If a user chooses to download from a third party app store, they are doing it of their own free will and they alone assume the same risks as browsing the internet normally. At that point it's really just internet 101 that any competent person should understand. Anyone else can simply choose to stay in their walled garden of safety.

Also, for the most part, developers won't develop for a third party store or make their own launcher unless there is a significant advantage to do so, like being forced to pay exorbitant prices to a monopolizing company in exchange for a false sense of "privacy and security". Apps aren't going to suddenly jump ship and make their own launchers. That costs a lot of money and Apple has curated a nice ecosystem. But, both developers and users should be allowed to choose what and how they install software on their personal devices. Android has proven that the wider user base will see virtually no impact and device security wont suddenly be compromised unless a specific user chooses to compromise their own device.