"Default MRI" (i.e. fully sampled) should definitely be acquired when possible when testing this out to compare to gold standard. But the benefit of using ML methods in the fully sampled case would be minimal (maybe some denoising), whereas they have a much larger effect when acquiring highly undersampled data that traditional reconstruction methods fail at. It's also not always possible to get fully sampled reference data. For example in functional MRI you might not be able to get matched fully sampled data because the benefit of undersampling is in improving the temporal resolution. These cases are definitely more researchy and less clinical though, and in my work we add a 2 minute highly undersampled scan to current standard protocols and compare what we can reconstruct from our 2 minute scan compared with the fully sampled (but often lower resolution) standard scans.