I guess that with discord (and many other non-foss free projects) the problem is that they start as free and then wanted to start to make money at a later stage.
For-profit software and companies are not necessarily bad, but they are bad when they take their existing software and start radically changing it for the sake of making more money.
If for example discord always had some features just for Nitro users and others for everyone, and those features (and the nitro price) would have always stayed the same it would have been much better