I mean, their strategy was awful. They replaced the start menu for the first time with a whole-screen abomination that ruined power user workflows. They made the desktop TOO much like a tablet, it seemed like they wanted one interface for everything.

Apple is very clear about their product differentiation and would never make macOS and ipadOS exact copies. Case in point: when they brought cursor support over, they painstakingly engineered it to work differently and snap to objects on the screen. I think if they did OP's suggestion and had a macOS screen show up when an iPad is connected to peripherals it would actually work out well.