While Successive Approximation is generally an accurate approach to Analog-to-Digital conversion, it is not a panacea. Its main flaw is in the fact that the D/A converter within, will eventually show inconsistencies. When that happens, some of the least-significant bits output will either be an overestimated one, followed by nothing but zeroes, or an underestimated zero, followed by nothing but ones.
Although circuit specialists do what they can to make this device consistent, there are quantitative limits to how successful they can be. And, whether 24 bits can be achieved depends mainly on frequency. In analog circuits, voltages tend to zero in on an ideal voltage exponentially, even when there is no signal-processing taking place. So the real question should be, ‘Can 24 bits still be achieved, far above 48kHz?’
And, if we insist that the low-pass filter should be purely numeric, we are also implying that one A/D conversion must be taking place at the highest sample-rate, such as at 192kHz, while if the low-pass filter could be partially analog, this would not be required.