Damn, they're actually a lot alike. The differences I see are subtle - Stimulus seems simpler in practice, scopes controllers, state stored in document, uses native APIs, etc. but there's definite overlap.

I said the same thing and noticed you had said it 4 hours earlier.

Whatever happened to knockout? If it failed, why?

It's not dead, it still receives updates (as recent as 4 days ago). There's an overhaul in progress called TKO (https://github.com/knockout/tko) that is supposed to bring Knockout up to date with ES6, a better plugin API, and other nice stuff.

My company still actively uses it in production and development

Yeah was my initial thought. I suppose Knockout is perceived by many to be old/legacy, though. On the up side Stimulus might help Rails apps minimize adding piles of jQuery.