Just let Calvin & Hobbes stand as its own beautiful finished thing. One of the reasons Calvin & Hobbes is so timeless and beloved is that it's not _that_ anchored in any particular time period. While it's dated somewhat by its big chunky TVs and landline phones, most of the "Calvin being a kid"-ness is timeless. Hobbes & Bacon couldn't make it five panels without referencing contemporary politics.
I agree with you 100% - it is done and Watterson has earned his retirement and then some, it is just the irrational, insatiable part of my psyche - you know the one that wants just one more chocolate covered almond, one more crisp, one more beer - that part wants one more. Just One More - that’s the story of our gluttonous journey through time.
I know this feeling very well. But after seeing so many things I liked slide into substandard mockeries of their peaks, I'm more than happy to watch people draw a line around their work and say "this is done".