I speculate that an earlier version of the story was that Śaṃtanu wanted a marital alliance with Vasu “Uparicara,” who belonged to a rival branch of the Kuru dynasty, established in Śuktimatī (Cedi) and controlling territory that extended from Magadha in the east to Matsya in the west. How would Śaṃtanu have held on to the Kuru kingdom in that neighborhood? Satyavatī was Vasu’s daughter and obviously Śaṃtanu’s marriage to her would have been a deal to unite the interests of the two Kuru branches.
Harivaṃśa tells us that Kṛtaka, son of Cyavana, had been chosen by Śaṃtanu’s elder brother Devāpi as the beneficiary of yajña. Vasu was Kṛtaka’s son according to the genealogies found in Purāṇas and some manuscripts of Mahābhārata. So, Śaṃtanu’s own claim to rule an independent Kuru kingdom was shaky unless he married the granddaughter of the king whom Devāpi supported.
Satyavatī’s beauty, ferry-boat, fisherman adoption etc., Śaṃtanu telling Devavrata that he would like to have more than one living son, i.e. a royal wife of childbearing age, and forty-year-old Devavrata’s vow of celibacy may all be details that were added to romanticize the story of a political rivalry.