Most users won't leave the App Store (be it for technological inexpertise or being used to it), so leaving it would mean losing access to a majority of users.You're asking the same question that gets asked and answered dozens of times in the threads on this topic.
Personally, my primary issues are developers that may choose to leave the App Store and piracy. For example, if Microsoft chooses to set up their own store, I will have to install their store for work reasons. So the choice to "simply stay" isn't that simple.
Besides, if you look at Android, most apps are on the Play Store despite it being an open platform.
And in the unique event your app gets taken off the App Store, you can just use an alternative.
For the piracy thing, I'll let you know that, on iOS, piracy is a thing even today without access to third party stores.