People are perfectly free to use Apple App Store and stay free from viruses and have a nice curated environment. Nothing changes for them if you allow sideloading. The ONLY change is that you have the OPTION to run software Apple doesn't officially sanction. That's just a basic freedom right there and it's hard to see any rational argument against it.

The rational argument is that the ability to side-load applications would be exploited by the nefarious and the clever to take advantage to naive users.

If a website can convince someone to download a "new version of the Flash Player", those same websites will convince someone to "side-load this application to protect against identity theft!"

Android allows sideloading and this doesn't happen (too much) in Android. It's possible to support walled garden by default while giving the user advanced options to sideload, which are difficult but not too difficult to find.

(Too much) and (not at all) are two different things.