Some collections are created by publishers but some aren’t and it would be a total mess if we would split some data to series and some to works based on this criteria. Most of the other databases see no difference between collections and other type of works. Some services linking to MusicBrainz provide only link types for works so for example from IMSLP or wikidata I can’t link to MB series but I can do that with MB works. I guess it’s lesser confusing for everyone if we keep all work related data in work entities (except work catalogs).
I think the situation is quite clear if the correct attribute is used with parts. If people just follow “Use this to indicate the parent work is not intended to be performed as a whole, but is just a grouping of pieces or works that can be performed individually” it should be clear.
This is off-topic but I must mention how amazing work you’ve done with Bach. After tens of thousands of edits during the years there’s now only 4 pages of recordings without performer credits. So no community cleanup for Bach because soon there’s nothing to fix