iPhones and most Android phones already support connections to external displays with a cheap adapter.

Android phones already allow sideloading. Even the much protested upcoming restrictions to sideloading don't interfere with ADB sideloading.

No hackers needed. This has existed for years. I've had a few friends go through periods where they thought they were going to use their phones as plug-in terminals for something and then they never last very long. It's never as satisfying as imagined.