Namor is much older, has fought in WWII, and has recently attempted wiping out a country (even if he wasn't in full control). If anyone was going to understand where this path was heading it was going to be him.
T'challa may have access to the knowledge and wisdom of past panthers but essentially they are just advisors and he ultimate acts based only on what he knows. Namor has decades of lived experience and makes choices based on that.