I'll bite. Why do you suppose it's an emotional decision? And why do you assume they're rewriting things and not writing new code in support of their business model?
> As our business grew, so did our resource utilization and latencies; to keep Schemaless performant, we needed a solution that would execute well at scale.
That's the third sentence. Clearly it was not an "emotional decision" and it was in support of their business model (scale).
That's built on top of Mattermost which is open source. They do build their own infra instead of using cloud though so that probably uses a big part of it.
Looking at the stock, they probably should focus on building new things/better the business model, rather then rewriting things in Go.