My sources are very simple: computer music server, and CD/disc transport. For the output I'd love AES EBU, but I'm fine with coax SPDIF instead since my run is only a few feet and I've experienced no problems with my current setup, which goes SPDIF-to-AES.
I think the market for such a device is small because most DACs already provide this functionality and have USB and SPDIF input.
I can see how active monitors may not have USB input and only have SPDIF or AES, in those cases such a device would help. But as you alluded to already it is very easy to replicate that functionality with a USB to AES/SPDIF device and some sort of AES/SPDIF switch.
The more specialized device the less of a market for it. For example I need to get HDMI from an AppleTV into an AES input in my Okto. I use an HDMI extractor + TOSLINK to coaxial converter (could eliminate this with an extractor that had coaxial output) + SPDIF to AES transformer. Would I like an HDMI to AES device? Sure, but I understand the use case is pretty niche.