I'm sorry, but much of Dropbox’s back-end infrastructure is historically written in Python, not Go. Go is a new addition to the stack. Only a small and very specific part of Dropbox is written in Rust, where things like an effectiveness of CPU caches becomes important. Other back-end code is still being written in Go.
While you're right that it's a small and specific part, it's also the core of the whole thing, so it's a very significant part. Dropbox isn't moving away from Go generally any time soon, as far as I know: it's still very much the default.
I guess I call "a new" what you call "a long time now". Dropbox is using Go for about 3 years.
To my knowledge Dropbox's backend is migrating (slowly) from Python to Go (new default), with Rust for some specialist features where the memory management model is preferable.