Maybe the issue with Linux on the desktop was because for a long time people 'including experts' kept saying year xxxx is the year of the desktop, then it didn't exactly happen.
Maybe the Hype or the expectations generated from that was too much.
As you point out what HAS happened is that Linux has come in to dominate the backend of things, servers, phones, cars, set top boxes etc, and Linux is just the kernel. Where as the front end can be bash, xorg, wayland, KDE, GNOME, Xfce etc.
The fediverse has now matured to the point where it can be used, and is used by anyone. But we don't have the media sharing that year xxxx will be year of the fediverse, so there is nothing to accuse us of making empty predictions. We can just get on with building what we have.