Note that neither CockroachDB or TiDB use Golang for their actual storage engine, which is in both cases written in C (RocksDB). They do use Golang for SQL parsing though, which is what this post was mostly about.
There's also DGraph based on Badger as a storage engine which is an all-go stack.
And badger does compare favorably to rocksdb under certain workloads.
We do, now. We're looking to move away from RocksDB to https://github.com/cockroachdb/pebble/.
[1] https://github.com/VictoriaMetrics/VictoriaMetrics
[2] https://github.com/klauspost/compress/tree/master/zstd#zstd