I agree with your point, but with an important caveat.
You are right that application developers should not be ignorant of databases, and DBAs should not be ignorant of what application developers are doing. All too often, this is the case.
However, the point when people came to think of application and database development as two separate things was the origin of databases themselves, and arguably one of the most important historical points in computer science itself. The notion that data has a life outside of any given application, and that it deserves itself to be managed, was and is incredibly powerful.
You are right that application developers should not be ignorant of databases, and DBAs should not be ignorant of what application developers are doing. All too often, this is the case.
However, the point when people came to think of application and database development as two separate things was the origin of databases themselves, and arguably one of the most important historical points in computer science itself. The notion that data has a life outside of any given application, and that it deserves itself to be managed, was and is incredibly powerful.