The argument isn’t that one brand can’t be associated with products in more than one field of endeavor. They can and there are many that do this. But the question is what result do they get from it? You say the Microsoft Surface has been successful; well, that’s your assessment. I’m looking at how much more successful they could have been if their purpose was clearer and properly communicated via their identity. If Microsoft builds another brand, it will be fresh, exciting, and it’ll open them up to new classes of people and their purpose for each product would have been clearer. I believe they could have used the venturing into hardware to achieve this.
To stress the point further; there are people who have negative buyer notions about Microsoft and will be immediately turned off by a “Microsoft” laptop. But with a different name and brand, they will actually consider it whether they want it or not. And if they figure out they don’t, it doesn’t matter. If they realize they do and later find out it’s from Microsoft, that will begin to change their perspective about Microsoft that they can do something “this good”