You're missing that you're a technically skilled individual, and most browser users are not. The design decisions must cater to the unskilled, because they will

You're missing that you're a technically skilled individual, and most browser users are not. The design decisions must cater to the unskilled, because they will not opt in manually. By default most OSes will use their ISP's DNS, and this will very often be insecure and compromised in some way.