I agree, this feature should be implemented.
Right now I’m building a site with a 2-parts menu on desktop (big main menu on more prominent place and the secondary menu in a smaller topbar), which should be joined together on mobile.
Both parts of the menu are populated using native WP menu builder and it would be much better, if I could use both of the menus as top level items both on the desktop and mobile.
I have to say, that I CAN insert Nav Menu element inside Nav Nestable and it seems to work, in a way. But the settings ov Nav Menu and Nav Nestable are conflicting and the native mobile menu is completely messed up (which I currently don’t care of, as I have done it by OffCanvas).
To sum it up, the Menu builder still needs some improvement.
And by the way, using the Nav Menu element as a second level inside the Dropdown, has some issues as well, as I already reported here: WIP: Buggy Menu Preview When Classic Nav Menu Used as Dropdown Inside Nested Nav
I hope it will be improved soon…
Thanks