What practical benefit would you achieve with the “unfold all” feature? Your example is limited to an empty base plate containing a single, simple model. In this case it will save a couple of clicks at best. In all other cases, and when working on games, “unfold all” would result in a mess that makes it much, much more difficult to navigate the explorer.
“Fold all” would have more practical use, but seems most useful to help clean up the mess “Unfold all” creates.
If you’re looking to dig into specific parts of your model, you can Alt + Click on a part which will then reveal it in the Explorer, no matter how deep it is. This will only unfold the models/folders that are relevant at that time, allowing you to find objects in the Explorer without making a mess.
Alternatively, in order to close all the unfolded models/folders above a selected instance, you can repeatedly press the Left Arrow Key to Fold the selection’s childs and move the selection to the Parent object.
I might be a simple developer, but I feel like these functions provide more functionality while avoiding the mess.