I think some people from industry would disagree. Certainly L4 has found use cases mostly in the field of embedded systems, but I wouldn't call this a niche (or at least it's a pretty big one).

Companies like Hensoldt Cyber (https://hensoldt-cyber.com - Gernot Heiser is involved in this) but also SYSGO with PikeOS (an independently developed L4-family microkernel - https://www.sysgo.com), Kernkonzept (https://www.kernkonzept.com) or Genode Labs (https://genode.org) have successfully used and deployed L4-based systems with many customers from industry. Yes, this is again very much focused on Germany, but that's also where a lot of the research was done.

Fuchsia at Google is probably a result of the "not-invented-here" syndrome that seems to be popular at Google. I haven't taken a closer look at the system but would certainly be interested in reading about the differences to an L4-based microkernel.

And Hurd-on-L4 is certainly coming next year ;-) (or a bit later, due to Corona or so...).