Is Plex now required for Infuse to work? I was planning on NOT continuing with Plex at all.
[Edit] NVM, I turned off my Plex server. simple solution, question answered.
Nope! 
The new Plex integration added in 5.7 is available for those who want to use Plex, but you will have access to all the same features whether you are using SMB, NFS, FTP/SFTP, or WebDAV, cloud services like Dropbox, Google Drive, OneDrive…or Plex.
The only case where features would be limited is if you are streaming via UPnP/DLNA, as those aren’t compatible with some of Infuse’s library features.
Thanks, James. I edited my post before I read yours. Turned off the server, question answered