Unpopular opinion, but more DEs is fine and good, especially if they will have teams that are either (a) large or (b) well-funded. Plasma and GNOME are very good, and Unity was actually great to use in its heyday.
Imo what we don't really need more of are the conservative, under-resourced 'classic look and feel' DEs like most of the minor players in the space. Those tend to end up incomplete and ill-performing, and there are already lots of them. I hope the existing ones thrive, but I don't think having more f them would do much good.
But anything as good as the big two, but with a different focus? Let's see it!