Others have already pointed out that pretty much all your point are wrong (1 is also wrong incidentally, discord is not a phone call, and wrt 2 nothing prevent moving discussion off of discord if it turns out to be a large undertaking, that doesn't remove the value of the simpler and more accessible channel as first step), but aside from that sometimes you want to ask a question about a question e.g. you've noticed something you find odd and have not seen whether it was normal or not, or you think that something is missing but don't know whether it's considered in scope for the project.

Opening an issue is a huge signal, putting in little investment means the issue is likely to be closed with barely a look, and putting in a huge amount of investment is a waste of time when the issue gets closed as out of scope.

Having a less formal channel with lower investment from all parties is useful.