Another great thread, describing this dilemma is "Don't build your castle in other people's kingdoms" https://news.ycombinator.com/item?id=29108662
In discords defense: It was never made for open source projects in mind, but gamers and small private communities. Having a free service provided might come with traps and pitfalls, but I would prefer that over having only paid online services. Choose your poison. I don't think discord is the right choice for FOSS communities.