You forgot to mention, that filename entry field is also in the middle of titlebar. While that might help to defend placing "Save" button next to it on the righ

You forgot to mention, that filename entry field is also in the middle of titlebar. While that might help to defend placing "Save" button next to it on the right, but now all primary save dialog functionality is contained in titlebar. Folder selection is clearly separated and made secondary action. It would be interesting to see usage statistics, how many users save files without selecting folder first.