I really think the idea that the riverine dialects persevere more archaic features is something that has to be scrutinized. I use to accept it uncritically. But isn't it weird people talk about northern somali being influenced by Arabic or other languages when basically almost every foeriegn pouplation that moved to somalia lives in the riverine region. How would it be possible for a place where you have literally hundreds of thosuands of bantus, arabs, Benadiri, bravanese and assimilated oromos to persevere a more archaic dialect.
We really should start with the assumption that these riverine dialects are all highly divergent. Its clear in the Arab example how nomadic pouplations would preserve a more archaic form of the language since they had less contact with outsiders.