As almost all Rule 5 picks are,
Song has a lot of potential, no doubt. He was viewed as one of the best pitchers in the draft, 4 years ago. Despite the fact that many top ranked pitchers don't pan out, many do.
Whenever a GM adds a player, other than by draft and IFA, where less information is available, they pretty much base it on the "knowns." It is very rare anyone is selected without even some AA action, and usually it's promising AA action. It's also very rare a player is selected, who has not played for 3+ years, and usually in those cases, it's a player who was proven before going out on an extended injury period.
I'm bummed we lost a promising pitcher from our system. It was a freaky confluence of happenings that has created this debate. I think this whole thing surprised even DD.