The problem with this approach in the real world is that multiple linear phase-outs of support from different programs might individually look "progressive" but

The problem with this approach in the real world is that multiple linear phase-outs of support from different programs might individually look "progressive" but in practice become poverty traps together e.g. if you're on one program because you have a disability and another because you've got small kids. In an ideal world, somehow all the major programs are carefully co-designed to not do this, but given that they might come from different levels of government, have been designed at different times and have their phase-outs "legislated in" in ways that's hard to fix.