Would it make sense to optionally also have a service that exposes these cameras through v4l2, so legacy applications can still make use of them, without patching them to make use of libcamera?

edit:

This would also resolve the dilemma of having complex camera drivers either in kernel or in a library. It could be in userspace while the kernel provides a small kernel module to make it work (like v4l2-loopback, or not unlike FUSE for filesystems).