That could end up being a two-headed monster, with some apps working fine in ‘iPad mode’ and looking awfully out of place in ‘Finder mode’ and other ones looking good in ‘Finder mode’ and bad in ‘iPad mode’.

Unless (almost) all their apps (including third-party ones) would seamlessly switch from a mouse-driven MacOS UI to a pen/finger drive iPad UI, I think most users would be disappointed with that.