If the usb in DDC is superior to the usb in your streamer you have a pretty poor streamer. As for I2S in general, many of the best streamers don't even offer it, they concentrate on usb and in some cases AES/EBU.
I agree with deep_333 in that downside of ddc is separate box, more cables, more parts to travel through which may have their own sound signatures, greater probability of noise being introduced, this goes along with the straight wire philosophy. The solution is to have both streamer and dac with optimized matching input and outputs.
I also previously owned Singxer SU6, it was not superior via usb or I2S to the extremely modded Mac Mini I was using at the time, this via usb.
As for I2S being the defacto scheme or output I've seen countless reviews in which people found usb, aes/ebu superior. Certainly it is understandable why in theory I2S should be superior (no extra conversion step, dedicated clock line), in practice this doesn't hold. I find it interesting that Linear Tube Audio Aero dac, an R2R dac getting great reviews doesn't even offer i2s, and you'd think this to be even more important with a ladder dac, the idea being timing extremely important to these relatively massive banks of resistors.