It seems to me that you think that the Discord would be “replaced” by the forums and everyone would have to migrate entirely to it. Why not just let the Discord and the forums co-exist?
You have a simple question about something and might want a quick answer? Discord.
You want quick feedback on something which isn’t worth to create a thread for? Discord.
You have a rather complex question which might require going into detail and you don’t want it to be spammed out of focus by other random stuff? Forums.
You want to present something finished or an ongoing project and maybe you want to have a rather deep discussion about it? Forums.
Also, just imagine being able to look something up on google and a detailed thread about it on this forum appears. Thinking about this makes me feel nostalgic and happy. Back in the day those old af threads helped me out much because almost every thread had an answer or at least an answer linked. At the moment, using the Discord search on the Garry’s Mod Discord is an absolute pain in the ass for me and probably many others.
This could be extended to an almost endless list of use cases and pro & contra arguments.