The best is so nebulous an idea in casting. There is no objective standard.
Someone may think someone is the best because of a vocal quality, or a life experience, or their culture, or their size, or whatever.
The idea of the best talent or the best person for the role can mean so many different things. It could be that Joseph Jones is the best person auditioning for the role of Desiree, but his gender makes him not as good as Mary Smith even though her voice is not as good.
Trolling as if there were objective standards or an even playing field or other factors affecting casting is starting to reach its limits.
If "the goal in casting should always be the best talent for a role, which means any outcome is possible" will we see more male Desirees and female Curlys? If they are the "best talent" available and that is the goal, well then you need to cast them.
But no one would (unless for a specific production like the Ashland Oklahoma), because other factors do play into casting.