Discord is great for a lot of things. It lets you ask quick questions and get quick answers by whoever happens to be online at the time, you can organize MP games on the fly, you can be in a lot of different communities and keep in touch with a lot of people. Sure, you won't get formal discussions in a discord server. But why does everything need to be like a forum? I don't really know why you talk about conversations being rushed on discord, that's just called a conversation and it's at the same pace as a conversation you'd have in real life.
Well, let's talk about what you've written a bit more. You are correct, via Discord you can ask quick questions and organize games and even have it up while gaming. That is where this tool is perfectly suitable for. I get no use of it as I don't do MP. Reasons: time and to some account toxicity on the chat. Really, I like to relax nowadays. Maybe I have become old - so what?
Being in touch with many, many people basically is being in touch with none - these might just be people convenient to know. That is what I learned from years of friendships in real life. A small number of people really matters and most are not really worth bothering or diluting your time on them. Was wrong already with legions of Facebook 'friends'. But then - to keep in touch with some base of possible players for a game of HoI4 perfectly fine.
The point with the 'not-so-formal'-discussions on discord. Well - they can even be toxic and frankly: I did not reach the age of 45 to listen to trash-talk or be target of such things. As stated above, I want to relax and tend to my hobby in a civilized surrounding without me worrying about people manners. Something I believe, Discord or other (voice-)chats do not provide me with.
Rushed...well let's put it like that: For me: Discord is chatting, a quick talk at best. A decent conversation - for me - is something that takes some time for thinking for every person partaking. I am patient, I can wait for answers. This is something, Discord does not provide me with either, I think. For me, things do not need to be told in 2 sentences from a smartphone. I am happy with reading some wall of text, if it is good. So obviously, I am not the target group and for me this is a wasted feature but maybe good for others.