I'm not sure any other technology stack would have fared much better.

Consider that 6 years, 2 months, and 20 days passed between Rails 3.2 and Rails 5.2. That's quite a bit of time for the framework to evolve. Then factor in the customizations from several non-framework dependencies and those added by GitHub.

This is an incredible achievement no matter how you slice it.

Yes, four FTE engineers taking 18 months to upgrade across two major versions indicates a massive problem, but not necessarily with Rails or Ruby. That's a cost of $1.5M give or take, just on the engineers, not including the opportunity cost in new feature development or paying other equally important tech debt.

$7 billion dollar company, $1.5M cost? $7 BILLION. Your order of magnitudes are waaaaayyyyy off.

This is the inverse of survivor bias, in that you are retroactively applying "best practice" at the wrong scale. What gets you from $0 to $7B may hurt you at $7B. Heck, may hurt you way earlier than that.

However, and YMMV on what problem you want to solve, but saving $1.5M, heck lets 10X it and call it $15M, saving $15M when worth $7B isn't the problem I'd personally be concerned with.

React 16 was a perfectly smooth upgrade for a massive part of the web ecosystem. It took 1 engineer to bump the version number and to test. Maybe a week or two at most to check that nothing was broken.

The React core team update tens of thousands of Facebook's components every time they update, I expect they'll continue to have uniquely smooth upgrades while that's the case.