I share the same frustration as you do.
My interpretation has been that playlists are seen as a feature to “lock in” people. If you make the effort to create playlists, you cannot easily switch to other players. Spotify is a good example. I prefer not to pay for Spotify, Apple Music and potentially Plex just for the sake of keeping my playlists. Just like my situation now with Itunes. As long as Plex does not properly support importing playlists, I still need Itunes and my Apple Music / Match subscription.
Once Plex supports importing playlists, people may also request it to export playlists, which may not be to the advantage of the developers. But well… I am not a developer and have no idea what’s truly behind making it difficult to import what is indeed just a simple text file.
In all fairness to Plex, it is already supporting the import of M3U playlists as an undocumented feature. @elan and @timwoj have mentioned this in another thread and @nosaj72 has confirmed it is working for him. However, it is not working for me and I have been posting my error logs above. It may be because I am running PMS as an Unraid docker and this feature may only be available in the WIndows implementation of PMS?
Did you give it a try yourself. It seems fairly easy to do based on the description below:
