Actually in some cases it can be reduced, atleast in some peoples view. Evert would probably be viewed better if she retired at 28 as a 14 slam winner, with all time records for consistency, who missed out on many more French and Aussie titles in the 70s due to not playing, then the women who got dominated by Navratilova when it was Navratilova peaking and Evert a bit past her prime. Connors often has it brought up that he has a losing head to head with all of Borg, McEnroe, and Lendl, when he could have retired after 1983 having won all 8 of his slams and all his other success, and have owned Lendl and more than held his own with McEnroe. Seles probably continued his relatively mediocre post stabbing career a bit longer than she should have, and alot of peoples memory of her is her being owned for years by the new big babes of tennis who had improved on her own power game. Even Agassi is remembered by some people for losing 8 matches in a row to Federer, despite that he was in his mid 30s and playing with a bad back. Agassi's last slam was the 2003 Australian and after the 2003 season completed it was pretty clear that was going to be the case, he had already cemented his longevity and amazing comeback to greatness, so was it worth playing another 3 years just to get owned by the much younger Federer and Nadal, injure himself further, and have almost no success winning tournaments.
Another reason it would be wise for Federer to retire soon.
