Well, OCaml, Haskell or maybe Scala would've been mature options in 2010, combining safety with language-level productivity that's comparable to Python, though the extent to which web frameworks available in those languages at that time were Django-equivalent is arguable. (Personally I would - and did - happily choose Scala with Wicket over Django in 2010)

In terms of what Instagram should do now, I'd say they should do what Facebook did: introduce thrift or similar, gradually move business logic into backend services written in more suitable languages, leaving the Python to eventually become just a thin web frontend. Retrofitting types onto code involves a lot of the same effort as rewriting it into a better language, and the rewards for the latter are higher, IME.