You are comparing a television show based on a comic book with a long running successful science fiction franchise that went through a lot to become what it is today. Again, there is no comparison. Assuming even if the Walking Dead franchise survives in another medium (outside of comic books) even after the television show was cancelled I doubt very much it would ever be as big as Star Trek.
Very few television shows come back on the air after being cancelled and even fewer go on to becoming successful multimedia franchises. Is there a Star Trek television series still running on television now? No. The last series started off with much fanfare and quickly succumbed to bad ratings and an inflated budget that quickly led to its demise. Combine that with the fact that fans got sick of the then current storyline and the reason fro the movie reboot was obvious.
Walking Dead's test will not come until a few years after the show is officially cancelled. That is when interest in the series will fall and most likely the price of the books will start to undergo a correction in price.
the only other tv series to air for many many years. get cancelled and not produced in years, then picked up again and is even more popular even getting good ratings in the united states that it did not do before , Dr Who,
there may only be a few comics published but the merchandising, toys ect is world wide