I like how after KDE5 it was still possible to use the older icons and styles from KDE4. Would be even better if we could also get to use the ones from KDE3. I personally cannot stand the breeze icons and style, currently use oxygen, and still find the old Crystal icons better, as well as the Plastique window decorations. I realize there are options in store.kde.org, but they are broken or don't work for plasma5. Since these were at some point "official" KDE icons, I think it would be nice to port them to plasma and include them in some "extras" package.
We do not have the manpower to maintain multiple icon themes or window decorations. Even Oxygen icon theme no longer gets new graphics, but only icon name updates. If you are not satisfied with the content of application store, please report it to the authors of the themes offered there.