I agree that the amount of information that can be held in the project Explorer is getting Larger and Larger as the product expands and at some point just as we did with View Filters (and will likely do again soon) we need to provide better ways to manage the data shown there and in the Project Explorer
You may have missed the Right Click Expand / Collapse buttons at any Tree Level - that allows you to Expand / Collapse all groups within a Tree Level or the entire tree or just the Utilities or Corridors etc.
I think Tabs could be a way to solve that going forwards, but I would see that as more User Definable than fixed - so for example if I am a Surveyor I may want a Survey Tab that has my Points and Geo-referenced images and Photogrammetry etc data on it. If I am a Takeoff Guy or Data Prep Guy I may want my Surfaces and Corridors in one tab and my Utilities in another Tab. So the ability to Right Click a Group Header (Utilities or Sanitary or Storm etc) and say Create Tab - Give it a Name and then have that as a set of Tabs at the head of the project Explorer - so If I am just doing Sanitary - I can create a PE Tab just for that while I am working on that to keep the PE really simple for that exercise - I could see those being permanent setups for one person and temporary setups for another. The Tabs could also be tied to a Menu - so for example if I am in Corridors Tabs of the Menu then it creates me a Corridors Tab in the PE - that way my PE is context sensitive and isolates just the data of that type (or maybe Corridors, Alignments and Surfaces for example) while I am working with that Menu open. Same for Utilities and Mass haul etc - so Menus could be a driver of UI behavior - however any degree of automation could also create some problems for some users.
You could also have a Work Process Mode selector that you could set up yourself that controls what is visible in the PE when you are in that Mode - if you created the Modes yourself and could decide what Groups of Data from the PE and VFM you would want to see while working - you could really customize your own experience and keep everything small in terms of Content - because typically you are using Layer Groups that are linked to e.g. Utilities Workflow or Sanitary Sewer Workflow etc and being able to reduce the PE to Sanitary Sewer and Surfaces and maybe Geo-referenced Images as well as changing the VFM to show only selected Layer Groups would be a great way to really minimize content display while increasing productivity as a result. You could also extend that to Menus as well - so that all menus you don't need in that mode could be hidden from view while working in that mode for example - that is kind of what we are all doing when we turn On and Off Menu Groups depending on what we are doing, or move menu commands from one Menu to another or from a Menu to a Sidebar to help increase our production - and I guess Wayne would likely want a different configuration for his Gaming Mouse for different types of work also ...
Just freewheeling here a bit because I agree 100% that when you have dozens of Layers (or more) and you have a full project with everything in it - there is a lot of info to wade through all the time (which is good in many ways but does slow you down if you do this all day every day
Good discussion to start here
Alan