They are two entirely different genre's of TV shows. I don't think you can compare a crime drama like CSI to a family drama like The Waltons.
CSI is an adult crime drama. The characters generally would not change much. The Walton as a family show with young children and older adults. The characters must change with the actors. Children grow up. Adults become ill or die.
The only time characters don't change much from the first episode to the last are cartoon programmes like The Simpsons.
"Do you realise how close you are to being spoken about in the past tense?" - Nora Batty to Alvin
"If I look at other shows, there isn't such a difference between the first episode and the last".
I could have easily said Brady Bunch or LHOTP instead of CSI.
There was a difference in the first and last episodes of Little House. In the first episode Mary and Laura were little girls 11 and 9 years old , Carrie was a toddler and Grace hadn't been born yet. By the end of the series Laura was a grown woman, a wife and a mother. Mary was blind and married and had left the show as had the rest of the Ingalls family and Laura and Almanzo were the lead characters and another family was living in the little house.
The Brady Bunch however didn't change much from beginning to end. At the end the kids were still kids just a few years older than when the series began and an irritating cousin was added.