Modern phones are more like general purpose computers than game consoles. The console argument from Apple is disingenuous and gets far too little pushback from courts. Same goes for their argument that developers who don't like the App Store rules should make web apps — but limits Safari support for PWAs and limits third-party browsers to an older, slower JavaScript engine.
From a different angle, corporations are not people; they do not inherently deserve the same consideration as people. Sideloading provides actual individuals the option of more flexibility in how they use the device they purchased with their hard-earned dollars. Sideloading also provides the freedom to continue to install apps that might be removed due to government pressure. "It's their platform" holds absolutely no weight as an argument in my mind; it's reflects excessive deference to corporations.
Apple should be forced because the real-world use of devices they make is broader than they argue in court, because it is a company not a person, and because other actions it takes restrict the ability of developers to take advantage of the alternative Apple itself promotes.
Less bureaucracy is the solution yet the United States, famously lazy about regulating tech, has managed to support only two truly viable mobile operating systems. Not even Microsoft wants to be in the game. This indicates that the bar is much higher than "they should just go make their own" and therefore we can expect more of the behemoths.