If you view a folder path as a kind of tag, then what you say is quite obviously incorrect.
Instead of dragging things into specific folders, drag them into specific tags. Same deal.
And then within each tag (you can view all files that have a specific tag, or set of tags; this is what the UI makes dead simple and fast to do) we can sort however we want.
The functional distinction between a folder name and a tag is really non-existent, except that tags have a lot of additional potential features such as: files can be in multiple tags; tag sets can be used to related data; tags can be applied to non-file data (e.g. people, or rather their contact / identity information), etc.
You mention the Dewey decimal system. You could apply such a system to your files using tags .. in fact, that's precisly what the Dewey system does: it's a simple tagging where everything goes in precisely one tag. This is completely doable with the system presented in Plasma Active .. you just aren't confined to it.