Okay but here's the kicker:

If you have an alternative OS like GrapheneOS, you won't be able to side load there either.

https://arstechnica.com/gadgets/2024/09/android-now-allows-apps-to-block-sideloading-and-push-a-google-play-version/

Google's Play Integrity API is how apps have previously blocked access when loaded onto phones that are in some way modified from a stock OS with all Google Play integrations intact. Recently, a popular two-factor authentication app blocked access on phones with modified firmware, including GrapheneOS, which aims to surpass the security of Android's stock system. Apps can call the Play Integrity API and get back an "integrity verdict," relaying if the phone has a "trustworthy" software environment, has Google Play Protect enabled, and passes other software checks.

Might look reasonable on it's face, but if I need to fix an app, or possibly get an app that was removed from Google Play, I'm blocked from that. What control I had over my Android phone is gone with Android 15, if I ever get that update. And switching to something else like GrapheneOS or Lineage, I'm blocked if I ever install GApps / GPlay.

Edit: grammar, clarity.