They did win the title with him last year, but Fisher's been obviously declining on defense for the last few seasons and LA refused to try and replace him or seriously upgrade their back-up PGs.
I think the real time the Lakers lost out, however, was when they swapped out Ariza for Artest. He's still a physical on-ball defender but he doesn't contribute much to team defense. Artest has never been a great offensive player and he's clearly fallen from his peak in that area. Most importantly, though, is how much athleticism the Lakers gave up when they let Ariza walk away. The only exceptional athletes on that team now are Kobe (who seems hurt and worn down) and Odom (who has returned to his normal inconsistent efforts). That means the Lakers are now almost always "out-athleted" at 2 or 3 positions for the entire game. It takes effort to overcome that, which is something these Lakers have never been good at.
Mike