As long as Google Play Movies has existed, there has been one major problem - you can't delete movies or TV episodes from your library. That might seem like a minor issue, since it doesn't cost users anything to keep content, but it has become more pressing since Google Play added support for 4K movies.
Play Movies has always sold content in both standard-definition (480p) and high-definition (1080p). Since there is no way to delete movies/episodes in your library, or upgrade them to the higher-quality version, you're always stuck with what you bought. Some users have been able to request upgrades by contacting support, but that's obviously not ideal.
As mentioned above, the lack of a delete or upgrade option has become a larger issue since Play Movies started offering 4K content. Many users have full libraries of 1080p films and TV shows, with no way to obtain the 4K versions, besides making a new Google account or contacting support repeatedly.
Hopefully this can be addressed soon, because it's becoming more of a problem than ever. And while you're at it Google, can you fix the Chrome OS app?