None of which require Apple to have a monopoly on app distribution.
Saying Apple users knew what they were getting into is kinda like saying Apple users agreed to the ToS, so they shouldn't complain.
What we should not do, is treating average Joes as idiots with no knowledge. They are non-tech, but not clueless. If you ask one of them about installing apps from the internet, they will likely remember that guy who told them it may be unsafe and it's better to stick with official store. PC users will definitely know about viruses and other "in the wild" hazards. Even my life-long non-tech grandma asked me about dangers and banking app practices when I brought her a tablet.
Personally I even bet that many of android users want/pretend apple safety or think that these two platforms are probably as safe as apple. They do not know what they are getting info, they assume that, because why wouldn't you.