The answer is in your question : "If this is not the case then please explain to me why you can still convey HD audio via multichannel analogue"
Digital HD audio is, in practice, protected, via HDCP because it's easier, because using HDMI. By "Mandatory", I don't mean by standard but by Major pressure. For easy reason, there is not such a limitation for analogue ...
There is no hardware limitation through optical SPDIF to route 7.1 digital LPCM audio :
16 bit 44 kHz : 6 Mbit/s exactly the standard specification for the Toslink bandwidth (IEC958 - early 80's).
Note this is an old specification, not a hardware limitation ...
So why was analogue component prevented from conveying HD video, but not analogue from conveying HD audio?
As I already stated, the same S/PDIF protocol is used to deal with HDMI's ARC capabilities and it still limits the audio to SD formats even though the HDMI interfaces used comply with HDCP. ARC doesn't use TosLink, it uses HDMI as the means by which it conveys audio.
S/PDIF is restricted to stereo PCM. Try streaming 5.1 SD LPCM via S/PDIF and see how you get on either via TosLink, digital coax or the Audio Return Channel of HDMI, all of which are governed by S/PDIF!
S/PDIF's inability to convey HD audio or multichannel LPCM has nothing at all to do with HDCP or hardware limitations. The protocol was devised before the conveyance of HD was of importance and as such was never developed to convey such audio. It wasn't developed any further because HDMI was adopted and because HDMI can convey both audio and video via the same interface and cabling. Future development of S/PDIF was abandoned hence why it can't convey HD audio.
Yes the answer is in my question . If you knew anything about HDCP then you'd know that it restricts the conversion of HD video to analogue, hence why the rhetorical question asking why it isn't doing the same in relation to the audio? The audio is not protected by HDCP or HDCP would start baulking every time you tried to convert digital HD audio to analogue in order to output it via multichannel analogue coax.
By the way I never mentioned any hardware limitations preventing HD audio being conveyed via S/PDIF optical so not sure why you are hinting that I did? S/PDIF is a protocol used to govern the conveyance of data, not a piece of hardware! Digital coax would be now struggling to convey HD formats, but optical can convey HD audio if given the opportunity. The only thing stopping optical from conveying HD audio are the limitations of the S/PDIF protocol used to govern it and there's no reason to update S/PDIF in order to facilitate this given the existence of HDMI even though HDMI is lousy when it comes to how it deals with audio in relation to clock synchronisation and jitter.