There are a few feature requests where folks go in to more detail as to why they’d like to see alternative database options for Plex:
Some of it is interesting reading. Some folks desire it for a perceived deficiency in SQLite3’s performance. Others want it because they’d like to be able to share a single database among multiple servers. Yet others just don’t consider SQLite3 to be a “proper” database.
I’m of the opinion that SQLite is perfectly suited to the task of being Plex’s database engine.