The wars (major) of the future will be won by technology, whether cyber, autonomous systems, AI, advanced weaponry etc. Soldiers have a place in that but strategically the others will far outweigh boots on the ground. A SoD should be talking about that, along with obviously looking after military personnel and just as importantly veterans, but I digress.
Likewise the SoD is the CEO, responsible for the long-term vision and strategic direction of the organization, along with governance and culture.
Comparing an elected official with an executive appointment is not the same, and really just a strawman argument.
As I said, he should get approved with all the others Trump has nominated. We should, and I do, hope they are successful. That doesn't preclude me or anyone else from wondering as to their respective ability to do the job they are nominated to, based on their existing experience.
For Hegseth, he is well educated, rose to the rank of Major - which is about middle management, and has worked for conservative think tanks, non profits and political advocacy groups, wrote some books and became a pundit on Fox news. That doesn't speak to me as being "qualified" to run the DoD, and I'm not just talking about executive experience.