Was "Death Note" supposed to end at episode 25/chapter 58? A friend told me that the writers of Death Note wanted to end it after L died, but kept on because in Shonen Jump, it's not common that the "evil" guy wins.

Is he right?
I can't speak for the writers, but the decline of the series in general after that point makes it so that the show is simply better in a technical sense without them.

Especially, given how one of the single biggest problems is the ending episodes and the total break from Light's character, who would never have been content with only one backup plan. He was facing people he thought were almost as good as L, and so likely wouldn't have underestimated them by not changing his plans at all to accommodate for the possibility of deception after Mello's appearance. 

Given how adaptable he was against L, it seemed contrived for failure to lose because he suddenly didn't adapt.