Ryuk was bored, so he dropped his death note to the human world...
The death god was also bored and he asked ryuk about what happened in the human world...
At the ending, he went to the human world... He wanted to follow that Ryuk did as he thinks it's interesting.
Yes, I'm anti-L but it doesn't mean I would change the story in whatever I want.
If you read the sequel, the death eraser was introduced to the story

Maybe the author would revive the character 'Light Yagami' again?
I don't know.