I think a lot of people misunderstand how this will be abused. No one is forcing anyone to sideload, but as soon as it possible to do so people will be creating malicious apps with step-by-step tutorials on how to installs malicious program X, Y, Z to earn cyrpto or whatever. People will do it, then blame Apple. Moreover, enabling sideloading will allow people to more easily see what iOS will and won't allow and find new attack vectors, some of which may then be used to attack other devices.
I think sideloading should be allowed much the way it is in macOS with respect to system extensions, but perhaps take it even a step further. If you want to enable sideloading you have to login to the bootloader, explicitly enable Reduced Security, turn off SIP and enable Permissive Security. Things like iCloud and ApplePay will probably be disabled and they should allow AppStore developers to choose if their software should run or update in an environment that isn't signed. Sideloading needs to be possible but difficult and extremely explicit about how you are reducing the security of your system. macOS does a pretty good job of this, so Apple does already have a model. I'd love to have an iOS device that I can really use for experimentation and testing.