Teixeira took an oath to defend the US as well as its military secrets — secrets that are vital to US national security and the physical security of personnel serving overseas. In breaking that oath, doing so almost daily for a span of a year, Teixeira committed one of the biggest violations of the Espionage Act in American history. His actions demand strong consequences, both for himself and as a message to other potential leakers.
While the actions of Teixeira were wrong and misguided, they were not motivated by malice, but rather from a desire to connect with friends he had made online, borne out of his autism. He is a youthful offender that has his whole life to look forward to. A lesser sentence would be more appropriate and would be more than sufficient for him to mature and pay the price for what he did, responsibility for which he has already acknowledged.