After my talk many people voiced same concerned of overhead. But I don't see it that way, overhead is not much over simplicity and familiarity benefits.

JSON has advantage of human readability and for that tooling support is required which probably you will loose with RPC. For example I use many tools to inspect requests that won't support it with RPC

This makes the assumption that their bottleneck is the overhead of HTTP. This assumption is common on HN, but (in my experience) is a rare problem to have for most companies.

It looks like they do a good job of tackling performance problems as they see them, even at the architectural level. My guess is they didn't tackle this because it wasn't a problem worth prioritizing compared to the compounded shitshow of a monolithic Rails app.