GNOME and Plasma are so far separated that a merger would be impossible, without either eliminating one of the two or completely rewriting both, and I think they cover different niches. GNOME is for people who want a tightly integrated experience, and KDE is for people who want to customize their system. (I would also argue that it’s not possible for there to be only one distro or DE, so long as all the components are open-source. Savvy users will always make their own stuff if they’re allowed to.)
There’s already plenty of cooperation between GNOME and KDE devs on common standards, support for each other’s apps, etc. I hope this continues, and makes both desktops better. A lot of behind-the-scenes stuff, like Wayland extensions, could definitely become shared between the two desktops.