What happened to consumer choice? There is a phone OS that provides all of this things — Android. Who wants this? Competitors, hackers, advertisers, and developers who don't want to pay for access to the platform Apple has created. I don't hear demands for this from the average Apple user, and when people ask me what device they should buy, I explain the differences between open and a walled garden, and explain why I chose the walled garden.
The problem with allowing sideloading is that if it can be done legitimately, it can be done illegitimately. If sandboxing can be ignored, and not enforced by the OS, then nothing prevents a malicious app/add/message from compromising your device. Making sideloading possible decreases security. Having to allow apps that don't follow the sandboxing rules put the entire phone at risk.
If Android didn't exist, there might be an argument. It does, so those who don't like Apple can use it. That's choice.