That would have been my guess, but it turns out that GNOME contributors still use Planet as a way to keep track of the state of the project:
From: FOR GNOME CONTRIBUTORS: How do you find what people are working on?
Which means people inside the contributors sphere still care about Planet.
Increasing the visibility of PGO through social media is good for letting people outside the project keep up with it: the X.org/freedesktop.org community is aggregating blog posts from their Planet instance on their Mastodon account.
The main editorial direction to remember, though, is that blogs on Planet GNOME are personal, which means they can be used for any topic. This has usually been a good way to humanise the GNOME contributors community, but it also introduces the possibility of people not caring about non-GNOME-related/non-technical posts. This editorial voice is also why we don’t have project blogs on Planet GNOME: even the “project” articles (GTK, Shell) are author feeds—essentially, a second RSS feed for some people.