Why on earth would you drive a date picker from the server? No one is saying to do that. JS is absolutely fine to build custom UI controls. The form the date picker renders in doesn't need to be 100% JS though.

Use JS to patch over things the browser doesn't implement and let it handle the rest.

Ok, at least we agree on something. I think that's fine if your app is simple enough, but the more and more dynamic it gets, at a certain point, I think it makes sense to just render the whole darn thing in JS. At least for the DX.