Key takeaway for me:

"crash-restart durable storage is a prerequisite for most consensus algorithms, I have a hunch we’ll see stronger guarantees as Elasticsearch moves towards consensus on writes."

It seems like ElasticSearch is inching towards a proper consensus algorithm, at least optionally, which makes me wonder yet again, why not just implement RAFT? While I won't speculate I will point out that the answer for other systems appears to have been related to ego (ie, distributed systems are easy).