Is there any way to just show the Evolution menu bar again for customers?
Short answer - no, not really.
The long answer is:
Menu (navigation between pages) looks different for each of the Evolution layouts. One of the goals of this change was to make sure plugin pages look the same in all layouts.
The new layout is specifically designed to be very similar to the File Manager. Users should be able to quickly pick up the similarities and use back button to get back to Evolution. It is also possible to navigate away from the plugin page using the page search action.
Keeping plugins inside Evolution (looking like other Evolution pages) is not really possible without injecting code and extra logic inside the plugin page. This led to a situation when there were a lot of exceptions from both sides. I mean from Evolution and from plugin developers. And the problems kept popping up to the point when some plugin developers (for example Softaculous) started completely bypassing the Evolution frame and opening plugin in a new browser tab.
The current plugin layout (Evo header + plugins contents) is designed to eliminate the need of all workarounds and give full flexibility to the plugin developers. And full control for the plugin developer as well, Evolution no longer injects logic into plugin pages).
We understand this disrupts the workflow users were accustomed to. However, form the functional perspective, it is still possible to navigate back to the main Evolution view with a single click. If we were to add the menu to the new plugin layout, we would be forced to make the menu look different than it looks on the layout users navigated from. This means the workflow disruption would be just the same. Or even worse because users would need to adjust to two different styles of menu (one in main evolution pages and one in the plugins page).
Hope this gives a better understanding of at least some of the underlying reasons for this change.