Agreed, but the implementation would be fragile. Poser is very liberal in the way the user can install files. It even goes to the length that it by default automatically starts to search for files when a file is not found at the specified location.

Unfortunately the default way of arranging content, made mandatory by Renderosity, causes files to be spread over a very wide folder-tree. The structure of that folder tree is shown verbatim in the Poser library, which causes users to endlessly run up and down that tree to combine figures with their materials, supporting props and poses. To make things worse, the structure of the tree is decided by maker rather than by purpose of the content.
No surprise users re-arrange content files to have at least some structure in their runtimes.

The 'purchases' folder is a good thing, be it that I fail to see the difference with the old 'downloads' folder. For me it is a sandbox in which i can examine the new content and rearrange/rename files to my liking, after which I move everything to the runtime folder where i want it to be, leaving 'purchses' empty and ready for the next session. That final Runtime folder ay any time may or may not be connected to the library. . .

Obviously, unless there is a way to update the info Poser has about where files are re-located, Poser will never be able to reliably un-install the product.Therefore better not have he individual un-install.

Un-install all is another way to flush the sandbox.