Not sure if it's true that LSM trees are so dominant, it was a bit of a fad a few years ago during the NoSQL hype. They can be a good default for concurrent write-heavy workloads like analytics or logging, but they can be tricky to tune.

The good-ol' copy-on-write memmapped B-Tree is still widely used, even on newer key-value stores like redb (I am more familiar with the Rust ecosystem), and it claims to outperform rocksdb (the go-to LSM-tree kvs) on most metrics other than batch writes [1] (although probably biased).

LMDB is still widely used (one of the main classic B-tree kvs), and Postgres, SQLite and MongoDB (WiredTiger), among others, are still backed by B-Tree key-value stores. The key-value storage backends tend to be relatively easy to swap, and I don't know of major efforts to migrate popular databases to a LSM tree backend.

[1] https://www.redb.org/post/2023/06/16/1-0-stable-release/