>Accepting that this huge repository of history will be wiped out when moving to the new service (I suppose you could always log back in and scroll through it, but it's at least harder to access, and is separated from all your new history)

This is true, but one needs to regularly back this up elsewhere if you care about it. If you're not in control of it, it can go away in an instant; Discord could one day decide to ban your server or anything else, and then it's gone.