Thanks for confirming my fears - maybe by OS X 10.20.1 they'll have changed this
It really comes down to the cost of iCloud storage space. Currently, iCloud Photo Sharing is free - it does not count against a person's iCloud storage space (the free 5GB, or whatever they've purchased). Apple has reasons for wanting to subsidize sharing of photos ("Look at the pictures I took with my iPhone!"). Apple certainly does not want folks to use Photo Sharing to put entire 200GB Photos, iPhoto, or Aperture libraries up on the web for free. So I can understand why they don't want folks putting albums (and nested folders) full of images online with a single click.
iCloud Photo Sharing is a feature that pre-dates iCloud Photo Library - it's available to users of iPhoto and Aperture (which do not support iCloud Photo Library), as well as Photos. And it's available to Photos users, even if they do not use iCloud Photo Library. Effectively, today's iCloud Photo Sharing is a legacy feature, due for revision/replacement.
If someone is using iCloud Photo Library (which means they're paying for their storage), then Apple has nothing much to lose by allowing easier sharing of the contents of those libraries (along the lines you describe). I expect we will see that, in some form or other, far sooner than OS X 10.20.1. What's harder to guess is what features may or may not be available.
Today's iCloud Photo Sharing allows for collaboration - multiple individuals can add images and captioning. However, with the exception of that and Family Sharing (which simply allows for a single, shared Family Calendar and the sharing of iTunes content), iCloud is a single-user resource - it's not intended for business/collaborative use, and Apple may never want to take things in that direction.
I can see Apple allowing easier view-only access to parts of a person's iCloud Photo Library (working much the way you envision - select the items you want to share, click the Share button...). Let's say, for Family Sharing members, a Family Library containing albums/images contributed by all members of the group. Such a library would consume relatively few resources - the images would be stored in the various individuals' iCloud Photo Libraries, rather than a separate library. It would encourage use of iCloud Photo Library, which certainly would seem to be a good thing for Apple. But would Apple want to make such resources available outside the bounds of the Family Share? Harder to know - it gets back to the whole business use question.
There could also be bigger issues to address if those individuals were allowed to
modify the library's contents (edit/delete albums, images and metadata). A simple shared album is easy to control, and the originals don't reside in the shared album. Accidentally giving a person the power to trash your entire library... there would be lots of room for unhappy campers.
Also, Apple may be saving us from ourselves (or more accurately, saving our friends and families from our selfies).
