I have seen many requests to enable the USB port as an audio interface, but it’s been suggested that is probably not technically possible since USB port may not be connected to the DSP.
Cheapest solution is to get an entry level audio interface, and stay analog. Downsides are that routing the signal through entry level preamps andA/D converters, and reamping with these is difficult. To get S/PDIF and keep the signal digital and unaltered, and reamping easy, one has to invest more that twice as much to get a higher quality interface, with probably much more I/Os and higher quality preamps and A/D D/A converters, that someone looking for the cheapest alternative is probably not planning to use. Plus they are in general not super portable.
I assume that a super simple interface that has S/PDIF in and out only, and nothing else, could be made for considerable cheaper than even the entry level audio interfaces, and with a quite small footprint. It could be the ideal add-on to the profiler for those wishing it had USB interface capabilities. And with the release of the Kemper Stage, potentially a number of new users whose main focus is live usage and don’t already have an interface; or that would like to keep equipment as light as possible, and still be able to track guitars at the highest quality while on the road, would find such a product very interesting.
So there you have it: Kemper or anyone reading with entrepreneurial spirit, go for it!
(And if such a thing already exists, please let me know where! my research has been unsuccessful).