The scale where throwing more hardware to run your CPU-intensive Python part (and not the part that just wait on a DB, IO or other networked service - that won't change with Golang) starts costing more than paying developers to write it in a new language and incurring the downside of introducing another language into the stack, throwing away all the "tribal knowledge" of the existing app and so on.

Modern hardware is incredibly fast, so if you wait for said scale it may never actually happen. It's likely someone will win the push for a rewrite based on politics rather than an actual engineering constraint, which I suspect happened here.