In Admin>Menus>Create Menu, I suggest a 1-click option to initially create a new menu populated with Top Level pages. (One can then continue to customize the menu, but now one starts with a Top Level menu rather than a blank menu.)
Related idea: provide a choice in Pages box of Admin>Menus for "Top Level Pages". Then one could click Top Level Pages, Select All, Add to Menu. Another quick way to populate a newly created menu with Top Level pages.
The current "blank menu", to which one adds pages, works fine; but I usually look to begin a new menu with a Top Level list rather than a blank menu. This approach also prevents accidentally failing to include a Top Level page as one tries to build a Top Level menu.