Sometimes I wonder, wy they aren't in the papers anymore...
Honestly, I have to agree with the decision to pull Calvin from papers.
Watterson said that he "didn't want to run the wheels off" with "the same old thing." Childhood is a gold mine for funny or weird ideas, but it gets repetitive after a while.
Calvin stayed six for the entire time the strip ran. All the while his relationships with other characters never changed. Once all of their little peeves have been abused, Calvin didn't grow up. He couldn't.
I don't think Calvin was all too strange for a six year old boy (albeit perhaps with undiagnosed ADHD). He's been around for just long enough to see the world as it is, but not quite old enough to understand it. Thus, he imagines insane, epic adventures that seem all right in themselves, if you don't understand how things work.
We were all Calvin at one time or another, and we all grew up. Life would get boring if we stayed six all of our lives. Calvin made a good run, and stretched one year into a decade.
Calvin did not 'end' in a sense that the strip curled up and died. There will always be adventurous little kids, and we were all once adventurous little kids, so the strip connects to everyone, decades later.
TL;DR? The strip reflected real life, and real life doesn't last forever.
EDIT: It's probably a bad thing that I have such a connection to a comic strip...