Twitter and Reddit both want to get profitable fast - which doesn’t work, as both companies have never been profitable and don’t have any experience with actually making money.
The reasons behind this are different (Elon just wants to make some money after he bought Twitter, Reddit wants to be somewhat presentable for an IPO), but the effects are similar and make me question if we’re at the end of the "free corporate social media" era of the internet.
Discord has two possible reasons for that specific change. For one it wants to get more mainstream and therefore loses parts of what made it special as a social network (e.g. the special username system).
I also think Discord (over-)reacts to its spam/scam problem here and tries to make usernames more distinctive.