I am of mixed feelings on non-Shultz Peanuts - although I lean more towards your sentiments above.

Comic strips (and even comics) are much different than books.  A novel or even a movie is a point in time, concentrated experience. Comic series and especially strips are a "relationship" between the reader and character.  While an individual strip might be funny or not on a given day - I think the attraction to the strip is the power of visiting daily or in little "pieces".  This gives a sense of familiarity with the characters and they become much more than static cartoons and begin to live a life of their own.

Peanuts is one of the best examples of where this live was interconnected with the creator.  Peanuts IS Shultz.  Anything not done by him is an homage at best and exploration at worst.

People can't understand why the real appeal of Peanuts is to read it over time in books (or, in the past daily).  If some random person were to read a strip where Snoopy crashes as the Red Baron, or Charlie Brown actually kicks the football - it would have little impact.  If a long-time fan were to read the same strip - it would have significant impact.

In some ways, Archie comics are like this.  While they to appeal to casual readers due to their comedy, stories, etc.  They are great by themselves - together as a mosaic over time - they are even better! (in my opinion of course).

This is why I never recommend comics to my non-comic friends.  I only recommend complete graphic novels that don't require any time commitment to "get".

I think this kind of depends.  A lot of Wizard of Oz nerds consider a good deal of the post-Frank L. Baum books to be "official canon" for example.  It's all about perspective really, which is, in the end, opinion. If people think the other strips are legitimate enough, they will perceive them that way.

I could see the argument that his descendants just want to get on the gravy train.  However, I can't really blame them either in this horrible economy where corporations are too afraid to hire people while they sit on their big caches of money and resources.