There might be a(nother) case where this new mechanism won't work.
If you attach USB device while the system is suspended, on resume the usb-core would find it and probe a driver for it. That driver would try to load a firmware, but because it have never been loaded before it won't be found in the "cache".
Honestly, why is userland even involved in firmware loading? The whole userland shenanigan should be scraped and reverted to the old system where the kernel loads the firmware directly from the filesystem. If the kernel modules are accessible, then the firmware would be accessible too. (Use tmpfs or initrd as workarounds for the other cases.)