Well… that depends a bit on what use cases you’re trying to push into Plex 
Plex is not designed for that particular use case. From Plex perspective, the server has catalogued your original file and that’s its reference – optimized versions are there to improve playback on certain devices (e.g. for syncing them to a mobile device or to avoid transcoding your files in real-time) – while maintaining the original files to play in original quality on clients capable to play them in that format.
There’s other apps you can use to create lower-quality files to replace your original content (e.g. Handbrake).
That being said… there’s a feature suggestion to change this:
