Back when I was in high school, our teachers taught us that the theme of TSL was the individual vs. society. I think that theme could be further refined to say that in his book, Hawthorne depicts Puritan society in the beginning stages of decay. He also depicts a newer, younger social order (as typified by Hester) that combines greater freedom and greater moral uncertainty.
The Puritans were overly strict and repressive, but I think the new social order that eventually evolved into our America had equal parts of freedom and instability that appealed to Hawthorne and also troubled him. Hester's A stands for Adulteress, but you could also say that it stands for Artist and Activist. All three titles fit Hester well. I think he felt the same way about the new American society that supplanted the Puritan colonists: liberal and progressive, but also disruptive and rebellious.
I agree that the Puritan culture could not surive. People are willing to accept control as long as it is protecting them too. When the country was young and new, it was surrounded by danger.