I notice you started by a blanket statment and use a strawman that is forms in React, a framework known for being the worst for forms (if you don't know the right library). Overall a poor criticism of SPA as a whole.
Having an opinion on best framework and libraries does not make what I'm saying straw man. React is by far the most popular framework for building SPAs and from what I've seen everyone uses a similar-enough state pattern to what has been described. I would love to hear your recommendations though.