I'm not sure how you're defining 'stable' here, but I'll comment on another aspect of your concern.

Twitter started with Rails, and at some point decided it was more efficient to do an incremental rewrite on the JVM.

    I can rewrite the entire backend in a weekend in Python.

I'd bet it took Twitter a bit longer to replace their infrastructure, and they survived just fine.

Build in whatever is rapid, for you. At this very moment. Using your resources.

Keep things service-oriented, decoupled. It'll be easy to replace things one component at a time, if needed.

in summary..

I think it's a perfectly cromulent platform to build on in terms of speed and scalability. But, in case you and I are wrong, follow my advice about staying decoupled and it won't hurt as much.