I'm not so sure about unattainable. Many programs exist that serve the user in very different contexts, for example responsive websites, web apps. Or games that work on console, like Steam Deck, and PC. Or the Nintendo Switch, which can be used in handheld mode, with a small screen and battery, or docked, connected to a TV. Controllers attached or unattached.
Now, I can see problems too: docked and portable modes need very different performance optimizations. But I'm sure that software can handle this, for example, IDEA IntelliJ has power save mode, and OS-es also demonstrated that they are fine on portable and connected systems alike, like MacOS, Windows, Linux.
It's also not a problem that some things are not available in both modes. For example, Switch has games that explicitly need docked mode, for example, Super Mario Party. Yet both the game, and the platform is popular.
I see no reason why a phone couldn't be a mediocre, or better PC.