Yeah, you don’t have to, but then nobody is getting your app. Most people don’t have other stores and will look at you weird if you mention side loading. Samsung/Galaxy Store maybe, but even Samsung users mostly don’t use it.

So technically your don’t have to use it and your app is obtainable without it, but If you want to actually reach some users, you have to be on Google Play.

Google search was just declared a monopoly, when there are obviously other searched engines anyone can use. If that’s a monopoly then Google Play is a bigger monopoly.