Except that some of those Nickelodeon franchises have been around 15 years -namely, Rugrats.Shows like Spongebob and Oddparents seem pretty safe based on the fact that they're two of the most popular shows on all of cable TV and have been around 7 years and 5 years respectively.
Dora The Explorer is still in production as far as I know, has been on 5 years and is popular enough with the Nick Jr. crowd to spawn a spinoff (Go Diego, Go! - which is beginning year two and the top rated Nick show among preschoolers knocking Dora herself from that spot). Blue's Clues has new episodes this fall which will mark 10 years of that show on television.
Just beyond that (in terms of current and potential longevity) is Jimmy Neutron which is begins year 4 with a new special in July and is still among the most popular shows on Nickelodeon (after Spongebob and Fairly Oddparents)
Beyond that things get less solid:
Backyardigans is the 5th most watched show with children ages 2 to 5 and is in its second year of production.
Avatar: The Last Airbender is beginning it's second year as well. From what I understand, it has solid, but smaller (and slightly older) following.
The weakest of the bunch:
Wild Thornberrys ran from 1998 to 2003 and is pretty hard to find in current line-ups, if at all.
Little Bill is out of production, but still runs on CBS on Sunday mornings and as recently as a year ago was in the top ten most popular shows for kids aged 2 to 5. Definitely on the way out.
And if all of that isn't enough, get this. Nickelodeon ranks first in total viewers of all the "kids" cable networks getting twice as many viewers as the number two kids network.
Which network would be that #2 ? The Disney Channel. 
Let's just say there's a reason a Nickelodeon hotel exists in Orlando and that parents are paying between $200 and $500 a night to stay there (based on rates for the second week of June)
But this isn't Nick vs Disney, this is Nick vs Peanuts. I can only use my children and their circles of friends as examples, but I can safely say they only know the Peanuts gang for the hanful of holiday specials that still run. To these kids they're one trick ponies, not a franchise. Nickelodeon is a daily ritual. Mention Snoopy and they think, "the dog from the Christmas show", but mention any of the popular Nickelodeon characters and they fall to the ground, forget their names and pee themselves all while giggling like maniacal little rodents.
The only stats I could dig up showed that the Charlie Brown X-mas special was watched by 15.3 million people on it's last yearly showing (Dec 2005). Spongebob Squarepants and Fairly Oddparents on the other hand, draw between 4 and 5 million on their single highest rated airing each week!
And that doesn't count any of the other airings for the week - Spongebob airs 6 times every weekday and Oddparents airs 4. (and that number doesn't count the additional exposure on other Nickelodeon networks or weekends, just the plain vanilla weekday Nickelodeon)
The Peanuts specials run once a year and the X-mas special running in prime time scores just three times the audience of the Saturday morning airings of Spongebob or Oddparents (which kids could see as many as 30 or 40 other times during the week...and probably do).
I suppose the flip side of the logic is that the parents are more likely to identify with the Peanuts characters and they're the ones with the money. But what drives the decision? Is it more of the parents seeing a "Ride On, Snoopy" commercial and saying, "We should take little Tommy there!" or is it little Tommy seeing a Nick Universe commercial and telling mom and dad, "I want to go there!"
While I think the longevity thing is certainly a potential issue and I have no working knowledge on the costs or terms or such licensing, if it is at all an opening for CF to get the Nick characters into existing parks, they'd be stupid to pass that up. Heck, a Rugrats ride installed in 1991 would still be relevant. A Blue's Clues attraction installed in 1996 would still be relevant.
At the very least take the safe bets like Spongebob. It's in year seven and the third most watched show on cable TV behind only The Sopranos & WWE Raw and twice as many kids are exposed to the Nick characters each week in the home courtesy of their TV than are exposed to Mickey and the gang.
That's some serious stuff.