"No, new codes would (should) leave older BDs playable."
Right, but only if the player is updated (correct?). The media companies are making the assumption that people are going to hook their shiney new BD player into an internet connection. Why would people do this? It's not a computer. Most people don't read the manuals. They hook up the A/V cables, or HDMI cable in this case, and start enjoying their hi-def movies. What possible reason would someone have for hooking up their player to an internet connection? Most people just aren't going to go through the trouble, even if there's some "enhanced" content on the disc.
And here's another way to look at it. Suppose some hacker does compromise the keys of one BD player and those keys get pulled. So that model is no longer able to play future BD titles, correct? Even if it's updated (which we have to pay for), the hacker will still just find a way to compromise those keys. It's a never ending cycle.