But by how much and does it actually matter? It's also a pretty small example. The problem of complexity outstripping benefits only grows the more you inspect the frontend landscape.
My main point is just that the implication that JS is required otherwise the UX will be unbearable is false. It's also really easy to mess up the UX with JS and I see it constantly. I would take a fast form post over a cumbersome JS powered experience any day.
"Then you would have to maintain two code pathways in two different languages in two different locations if you need to update the validation rules."
Not necessarily. You can use the same exact code, in the same location written in the same language.