The question is wrong:
Is DRBD the only viable block replication solution for Linux?
I've found myself needing redundant storage at a block level.
No, it is not. You have e.g. Linux MD (software RAID), LVM RAID. They provide redundancy for block devices.
So you probably wanted to ask:
Is DRBD the only viable NETWORK block replication solution for Linux?
And then again you have other options.
If the client is one node only you can build software RAID on the client which replicates over several network storages.
If you have multiple clients you can use clustered LVM block devices.
File-level replication (Gluster, GFS, etc) does not work for my use case.
GFS( Redhat's GFS2) is a shared device cluster file system. It does not provide redundancy. Other local filesystems like BTRFS and ZFS can do it though. As well as other distributed file systems.