Apple already provides first party software to handle iDevice backups on Windows or Mac.
Backing up an Android device to a PC using adb is significantly more difficult, especially for the less technically minded.
That’s arguably the wrong question: I bet a lot more would own one if they could easily backup their iOS devices to it.
There aren’t that many people that are willing to own a device from a company but not trusting that company with their data
Some of the power amplifiers for rarely-used bands probably qualify as well (mmWave in particular).
On the software side I’d have to dig a bit, but I bet many code paths on iOS see use of less than 1% of all users.