v4l2loopback has highly scattered but at the same time wide base. It even includes stuff like scientific research. Here's one recent'ish example:
https://dl.acm.org/doi/epdf/10.1145/3664647.3681007. This also implies that it is a tainted driver likely deployed to multiple critical organizations and environments.
This type of feature is also quite robust tool for many uses where emulation is required but we still need sufficient figures for latency and throughput. It could be applied e.g., to drone platform testing where vision can be mocked with virtual devices providing simulated environment or pre-recorded environment (for training AI).
It is also good to keep in mind not every single piece of legacy software can be magically turned into PipeWire clients, and neither it is a great fit all possible deployments of Linux.