Not really. Microsoft could have built a tablet OS around lightweight mobile software that could have shrunk the size of their tablets considerably. But they chose to use Windows.
No. But I do think a lightweight tablet based on a mobile OS could have been built in 2002. Along the lines of a 7" Palm device. The Newton was released in the early 90s.
Maybe. I agree though that MUCH of what held MS back was legacy desktop, no argument there. But MS has a loooooong history of doing stupid things. They had windows mobile for a long time and basically ignored it instead of creating a true smartphone like Apple did.
In any case those technological hurdles are gone and having windows destkop, from a hardware point of view, does not entail any trade offs anymore. Software and OS are still up for debate, of course, but I mean strictly from a hardware point of view.