I don't think the series necessarily died but I did feel that L's death was very much the high point in the series. Near and Mello were much less believable as geniuses. Not that the intelligence of any of the other Major characters is believable in the least.
In fact, I kind of feel distasteful about the story looking back. There are two ways to pull off a genius characters in fiction. Either make a character that's a genius in his setting, or make a setting that makes a genius of a character. The latter is what Death Note (and Code Geass for that matter) does, and it's much less believable then the former. On multiple occasions I was shocked, not because the given character was so smart, but because Light, L, Mello, and Near are apparently all Kwisatz Haderachs. I mean, if you are going to give the characters powers that operate outside of reason, then at least go ahead and label it as such.