It's not only alternative app stores. It's browsers with ad-blockers, youtube clients that auto-skip sponsor messages in the middle of videos and apps for $0, that cost money on the main app store - although their code is open source.
Interesting, I'm generally very opposed to Apple's way, but this sounds like a real unexpected benefit (silver lining on the cloud maybe?). It enables what I've long asked for from devs, mainly that they make the source available to paying customers.
A software project is using an open source licence. And app gets made for Android, and because of culture or GPL, the app is open source, too. But on Google Play, the app costs money, contains ads, or there is sub-features, that you only get if you pay a (one time/monthly) fee.
But because it's trivial to sideload on Android, I can just get the app from github (or from the open source app store FDroid) and install it myself, for free. No ads, no locked content.
There are almost no applications (at least that I've found) where you can download the source code of the app, build it and then install it locally to your device (except for testing of course).
* OSMand, the main openstreetmaps client on android: the version on Google Play comes just with the basics, if you want extras, like the topographic height contour lines overlaid on your maps, which is important for mountain sports, you had to pay, even that data is free and the feature is GPL-3.0.
* Simple Mobile Tools, a huge software suite of little tools (address book, voice recorder, calendar, ect.): if you get it from Google Play, it nags you to buy the "thank you" app, even if you already bought it. They has also started releasing "Pro" versions of the tools, which have more functionality and for which you have to pay - again. Everything is GPL-3.0., and therefor free on FDRoid.