I don't think ZFS is displacing any of the existing file system options. It's just another option you can optionally choose to use. In my case I'm using individual ZFS formatted drives in a normal parity protected array.
The advantage here is that you can use snapshots, compression etc. But I retain the advantage of being able to use disks of different size and I can mount them individually. I also use ZFS on my cache drives because I don't love the way BTRFS reports free-space and have had issues with this in the past.
I can see why there are use cases for people wanting a full ZFS array too. If speed is a priority then this will always win out over a parity + cache combo. People doing media creation (e.g. YouTubers etc) may want to start with a parity array, then migrate to a full ZFS backed one later. Add in the paid support options UNRAID now has and you have something you can rely on as your needs grow/change.