What changed though? Before iOS and iPhones existed, you also couldn’t sideload on iOS, but no one was complaining. Apple created a new product essentially from scratch, and it has some limitations to how software can be distributed to it. Your car infotainment center and your fancy air purifier probably also don’t provide an easy way to install arbitrary software on them. But creating a new device that can’t do every conceivable task doesn’t change how you can sell other software online.