This is the reason it was changed from stream format, to a forum post, as given on Supercruise News.
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If it really was all about maintaining the integrity of the message, so it doesn't become diluted. Why not have the stream as planned, and then post a thread along side it with all the info given in the stream? Why not both?
By delaying CM talking about it until we, as a community, have discussed it, they can address our concerns regarding the development post. This makes sense...
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If you are saying to do a stream first, and then post what was said during the stream, then this is problematic. So... any attempt for them to translate dev concepts in a live stream to a forum post is prone to more problems than the devs just writing it themselves.
At the same token, devs are not the best communicators. So devs, sitting in front of a camera, walking through dev concepts with a largely angry, critical, and possibly confrontational audience is largely problematic. It'd be a lot easier for them to just write something ahead of time and let people whose job it is to be "customer facing" to deal with the customers.
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If you are saying you'd rather have the forum post posted concurrently with the stream, and have the stream go over what was on that post, I think that's fine...But the CM team would lose out on the community feedback (~ a day's worth), unless they're parsing/addressing our reactions/feedback in realtime (largely problematic and difficult). They have substantially less time to think about their responses.