I think there were seven people named Swett in Newbury in 1642: John Swett of Newbury and his wife Phebe; John, Jr. and his wife Sarah; and three unmarried sons of John and Phebe: Stephen, Joseph, and Benjamin.
John, Jr. moved to Charlestown about 1649. Sarah died in 1650. John Swett of Newbury died in 1652. Joseph moved to Boston about 1658. Benjamin and his family moved to Hampton about 1663. Phebe died in 1665. Only Stephen stayed in Newbury.
In 1678 there were eight people named Swett in Newbury: Stephen aged 58 and twice a widower; his daughter Rebecca aged 9, son Benjamin aged 14, son John  aged 30 with his wife Mary and their three children, Mary aged 6, Hannah aged 4, and John  aged 2. Stephen's daughters Hannah and Elizabeth were married, and his sons Stephen, Jr., and Joseph  had moved away.
In 1695 Stephen  Swett, son of Captain Benjamin, moved back to Newbury from Hampton and started a family. By 1730 almost half of the Swetts in Newbury were descendants of Benjamin. Slightly over half were descendants of Stephen through his son John  and grandsons John , Samuel , and Benjamin .
The total number of Swetts in Newbury began to decline about 1730 because a large percentage of young men moved away and established their families elsewhere.