Regardless, Microsoft would be putting themselves in a precarious position legally if they undercut OEMs in the way you suggest... even if it made sense to do it, which I don't believe it does. Undercutting the OEMs would mean less profit for Microsoft. It's a complicated calculation, and Microsoft does have to be careful here, but I honestly don't see any way that them making hardware means there's no room for OEMs. There is the chance OEMs would opt for an alternative (like Android) that would offer them a larger profit margin, but that's just more reason why it's not a good idea for Microsoft to give the OS away for free to customers of their hardware.
OK. Last time I checked, $99 is $80-$100. But Microsoft undercut the OEMs all the time with Xbox 1 and Xbox 360, selling at a loss to gain popularity. I don't see any legal issues in that. But I think that either MS should stay out of the hardware business, or pay the OEMs to build Microsoft hardware. That would give the OEMs incentives to build beautiful hardware.