Android is almost as locked to Google Play as Apple is to the App Store, to the effect that no vendor has made a successful business selling phones without the Play Store. While they've finally fixed it, for years, they used scare tactics to prevent sideloading by insinuating it as a security risk, requiring you allow "less secure apps" in order to sideload. Most applications won't run without Play Services even if you do sideload them, and Play Services can remote uninstall an app from every device anyways, should they choose to employ it.

Yes, you can roll your own with a ROM and limit yourself to the tiny selection on F-Droid, but you enter an increasingly niche area. While it's "technically possible", it isn't really supported well enough to justify holding it as a benefit over a competing platform from a mainstream sense.

Bear in mind, when faced with a technical question revolving around making a backup of APK files, a core Android engineer said they "didn't support piracy", which was... a pretty hefty accusation to throw around for an innocuous backup of apps for being able to sideload should they be pulled from the then-called Android Market.