In the latest WP the admin interface is changed and you have a popup submenu. I really liked the old version better with expandable menu. Is it somehow possible to get that back again for all users?
Yep, believe so. Try this (enqueue in admin styles):
Show sub-menus at all times
Hide Pop-up Navs
That should get you started with it, otherwise you can go to wp-admin.dev.css and edit the navigation there, starting at line c. 1519. Hope that helps..