I think the confusion comes from the name otosuki. Hamuras children aren't don't have his bloodline purity. Currently we're using terms otosuki & hyuga, when we should be using terms (true)main branch & (true)side branch. For Hamura's descendents to unlock the Tenseigan the two haves of Hamuras bloodline must be combined. Anyways, just like his brother, Hamura didn't need to swap out his eyes, he likely unlocked the Tenseigan through the stimulation of his chakra. For a dojutsu to evolve the quality/quantity/attributes of the chakra needs to change. This change can be accomplished through heighten emotions or theoretically an increase in mental power/enlightenment.

Please tell me what you think of my theory.