If the SDR DSP after the roofing filter is so effective, why does Yaesu deem it necessary to offer an optional 300 Hz CW roofer in addition to the stock 500 Hz roofer?
Superhet SDRs need roofing filters if located near a high power transmitter that greatly exceeds the dynamic range of the SDRs ADC (e.g. multi-transmitter contest stations, etc.) With a direct sampling SDR (no roofing), very often (but not always), adding some front-end attenuation, and/or some RF filtering, will allow DSP filter processing gain to work, even under high dynamic range conditions.
For instance, the 8-bit ADC in my inexpensive RTL-SDR v3 will easily get overloaded, when used for HF, by 2 nearby line-of-sight broadcast towers, blocking most all weak signals. But if I add AM and FM broadcast band-block filters, its sensitivity improves greatly when used in conjunction with narrow DSP CW filtering.
IMO, using a superhet SDR with a roofing filter only provides a clear advantage (over a direct sampling SDR) in multi-transmitter contest stations or neighborhoods or near broadcast towers, and in an otherwise quiet RFI environment. If you are not near other transmitting stations and in a neighborhood with even a moderate RFI noise floor, even a 12-bit direct sampling SDR will allow DSP software to dig out the weakest signals that one's antenna can pick up.