Discord vs IRC is just another extension of the "open, decentralised vs proprietary, centralised social media" debate. It baffles me that even people who are aware of, and opposed to, the ongoing enshittification of Twitter and Reddit would so happily choose Discord.
That said, I think privacy is a weak argument against Discord (or, rather, a weak argument for IRC). I have used IRC for many years and I never assumed that anything I put on there was private. Everything you post can be seen and trivially recorded by other users as well as the server operator.
To me it's more about not having your access to social media entirely at the whim of a corporation whose incentives are not aligned with yours and who, sooner or later, is going to come under significant pressure to monetise you. So I guess, in a word, control is the main advantage. This includes control over whether you can access the service, how you access it, what you can access and on what terms you can access it. IRC's simplicity is a definite plus in that regard.
And then you have the "average user", who ruthlessly selects for ease of use and cost and doesn't care about any of this stuff. I think that is a short-sighted position, but ultimately people will make their choice and the average user's preference for walled gardens built by VC-backed corporations is so strong I think it unlikely to be reversed by any amount of blogs or HN comments. I think these services (or their successors) will always continue to exist and will probably always be far more popular than the open alternatives, but it is still worth maintaining and promoting those alternatives, which can provide a sustainable platform for those of us who care about such things.