I don't work for Bose but it appears to me that Bose has said multiple times they are not going to provide a way to downgrade the firmware. I asked myself why would that be the case? There are probably thousands if not more of QC35s that are still on that firmware and many that are still waiting to be sold with older versions of the firmware. So why? I can only think perhaps that Bose technical people assume that there may be newer versions of the QC35 that never had anything but the current firmware and they don't want to take the chance of supporting those QC35s with older firmware not designed for those small 'engineering changes'. Personally I can't think of another technical reason - the rest of the decision has got to be more organizational and financial (maintaining software engineers to support the downgrading process).
So how many versions back of the firmware do people want? Do we actually know the differences between all the differences of firmware. At what point was ANC changed and are we happy with the previous version of ANC? There is mention here of 'Sidetone' - do we know if there have been any changes to the firmware at all with regards to sidetone?
And while I am NOT advocating anything that would put a pair of QC35s out of warantee or be illegal (as Bose firmware is copyrighted), there is quite a bit of technical talent in this forum - enough to take apart a QC35i and fix the randomly on issue which definately puts that QC35 out of warrantee. So I am suprised no one has reversed engineered the firmware and provided an alternative download. I see that happen all the time with my dashcam camera's firmware.