I can understand why we would use estar for temporary states. But there are conditions that people have that are not temporary, such as being old (or for some people, being fat).
¡qué gordo está!
¡pobre abuelo! está viejo.
¡todo está tan caro!
These subjects aren't going to change: people won't get younger or thinner, and things aren't going to get cheaper.
Why do we use estar for these and not ser?