While I fully agree that Trello could be js-free I don't think either of us would have known the name "Trello" to talk about today if it was.

I disagree. Very little (if any) actual functionality would be lost. The differences would be more-or-less cosmetic and (to a limited extent, though I think this would be positive rather than negative in many cases) ergonomic.

I understand that you can degrade your web app to the point it works without JS but at what (UI/UX) cost?

None if you design your app with progressive enhancement in mind. Drag-and-drop and other forms of "interactivity" could be added onto the non-JS version rather easily.

It's simply not worth the resources

It costs resources if it's not something you consider right from the get-go.

In other words, it only looks like it would use more resources because you're coming at this from the perspective of "okay, here's a JS-heavy app; how do we make it JS-free?" instead of the more efficient perspective of "okay, here's an app solely in HTML, CSS, and Rails; what JavaScript should we add to make this cooler?". The former is like trying to learn how to ride a motorcycle before learning how to ride a bicycle.

"Screen readers don't play nice with js frameworks" (which is not true across the board) are not an argument for no-js, it's an argument for building better screen readers.

I agree with this. That doesn't mean that the existence of Javascript can always be assumed.