There's quite a distance between "tiny" and "top 100 sites"! Some pretty darn popular sites that use Rails (afaik) are GitHub, Shopify, Hulu, Twitch, and Airbnb.
Not really. The web has become massively centralized with almost all traffic going to a small number of sites. The bottom of the top 100 list is only getting about 50 million pageviews a month, even poorly written sites in slow languages can easily do that on very cheap low end hardware. Pretty much anything not in the top 100 is dealing with a tiny amount of traffic, even a fair bit of the top 100 is. So it is github and twitch, neither of which actually do anything substantial in RoR any more. That seems like a pretty good reason to think RoR's strength isn't high traffic volumes.
This is wrong. GitHub is still a large Rails app. See presentations by the core Rails team people who work there like Eileen Uchitelle and Aaron Patterson.
That's what I am going on. It literally says the RoR stuff is just the basic webpages, everything else has been re-written or was never RoR. Same deal as twitch, the RoR is just the trivial web portion which is mostly cached. All the chat and video is in go.
This may be true of Twitch but not GitHub. GitHub uses https://github.com/libgit2/rugged to bridge between Ruby and Git. As far as I'm aware, they still use Ruby to pull the diffs etc out of Git etc.