Apple claims that allowing sideloading and alternative app stores effectively opens the door for malware, fraud, scams, and other harmful content.
And allowing your kids to play in the dirt may cause them to develop certain infections. These can be dangerous, but they're also useful in training immune systems and reducing future vulnerability. Not to mention that kind of "free range" upbringing promotes awareness, responsibility, and self-sufficiency.
I'd feel differently about Apple's walled garden if there were a half-dozen or more smartphone ecosystems. Then, if one or two were severely locked down I wouldn't care. But right now we effectively have two such ecosystems. Both are highly flawed, and both treat their customers simultaneously as dependants and as products.
But at least Google allows, (for some values of 'allows'), its Android users to install both alternative versions of the OS, and apps from just about anywhere, on the phones they paid for. Although it falls short of the freedom I'd like to see, it's freer than Apple's approach.