They don't want forum.bitcoin.org to still point to this forum (otherwise it would still be implicitly "official"), so there is no other option but to break the URLs.
Sure there is. One would be to simply not link to bitcointalk.org (as they don't now), but maintain redirection for old URIs. This will stop the promotion of bitcointalk.org and distance it from the official project without breaking hundreds of links.
If that's not good enough, another less ideal solution would be to display a page explaining that the URL has changed and bitcointalk.org is no longer the official forum of Bitcoin, then redirect in 10-20 seconds. The reason that's less ideal is that the redirection is done in HTML or Javascript rather than HTTP, so a little bit of semantic information is lost.
