There's no set of prerequisites for being an NBA head coach in which the most qualified or "most competent" candidate, from those eligible, would have been Tim Duncan. Literally, the only argument that can be used against Hammon that cannot be used in triplicate against Duncan is, "she didn't play in the NBA," and that isn't a job requirement.
Popovich, like Adam Silver, has a nearly impeccable track record, so he gets a mulligan or two. But he blew that one.