I think the concerns for opening up threads on the forum when it has been discussed on discord are valid. I believe for certain things this is certainly a must. But I have a few things to add:
1.Whether it is the forum or discord, no every builder+ or user participates on every single topic.
2.The discord server does have a section for Danbooru/serious discussion. I started going trough every message posted there even when I wasn't a builder. It's a more easily digestible way to know what is happening in all aspects of Danbooru (except technical, a separate section) and I don't believe I'm alone in doing this.
3.Cementing things that were discussed and agreed upon on Discord, is a fine thing. Just accept that it was discussed in a valid way there. And that the forum post is not much more than a formality/redundancy.
4.Discord has created for a me a more bustling community. It made me go from lurker/plat to builder. It encouraged me to join the conversation more informally and sub-sequentially more formally in the Danbooru section. I believe it has been touched upon how intimidating and closed the forum feels on this site because of the "serious business nature". Not that there is anything wrong with that.
Obviously I'm biased towards the use of Discord. Like I mentioned in my third point, cementing things on the forum is a fine thing to do. I'm just trying to get across that the Danbooru related discussions that happen over the discord server are just as valid/official as the ones on the forum.