Not really. The firmware is basically protecting itself - the closed source firmware contains proprietary image processing code (ISP) for the camera which Pi Trading paid for, so it's only supposed be used with the Pi Camera.

A complete open source re-implementation would either not support ISP, or would include a non-proprietary version of the same or similar code, at which point they shouldn't care.

There's also protecting access to the hardware video codecs, in order to account for licensing fees. They might care about that if the MPEG-LA starts being a dick about it.