downloading an APK and opening it is already about as easy as it gets. the only thing easier would be for someone else to do it for you
"This APK cannot be scanned and its safety cannot be verified. Learn more/go back" and "learn more" has a link that looks like nothing but is actually a button to actually install the app.
I can think of some easier things, for example popping up a dialog, pressing "install" and having my all actually be installed after that.
You're forgetting the installation ("sideloading", what everyone else calls installation) restrictions they are about to deploy. It will be a significant hassle to install anything without Google's approval. Many F-droid apps are showing warning notices about this upcoming change.