Long term flexibility is a very surprising way to describe a JS stack. They are easy to build in, but there is nothing robust about the ecosystem. When in 5 years a PM asks me to add that simple multi step form, I'll probably have to tell them it's blocked by replacing deprecated dependcies and our validation library doesn't support our new element types so we need to rewrite all the validation across the site to use a new library... man I feel a burnout coming on.

My counter-argument is that the simplest tool has the largest chance of not breaking over long periods of time. I have dynamic websites I built 10 years ago that still run almost untouched because I used the simplest stack possible.

Contrasting that, in the commercial software world we rebuild from the ground up in 1-5 years, and we aren't even building complex things. Same level of interaction, but a stack that has an enormous complexity and web of dependencies and moving targets that absolutely require a steady hand on the maintenance wheel. Your examplen scenario is not that scary, we have been building that kind of UX long before React and friends.

Your final comment about people who disable javascript is disappointing, but an opinion you likely share with many PMs. I would recommend a look into accessibility, and also try to see why people might advocate for graceful degradation.