Agassi's career was very atypical. He never had a dominant season (titles in the double digits, 3 slams, etc) and his entire career was the ultimate roller coaster but he hanged around much longer than anyone would have thought and surprisingly enough (for me at least), much longer than Sampras.
Fed doesn't fit that profile at all. He definitely had a prime (titles in double digits, multiple slams, masters etc) and a post prime with fewer titles. 2009 was great but still, 4 titles total is visibly different from his prime years. 2012 has been the only post-prime season that showed similarities to his prime years. I wouldn't be surprised if 2012 turned out to be Fed's "swan song".