It's like this, Duke Nukem is a great character, that is one of the reasons why people were willing to look at this whole "Forever" thing as a joke. He also hasn't been overused like a lot of other relatively iconic video gaming characters.
If the "Forever" thing died, I see no real reason why that means the whole IP would be dead as opposed to simply passed to someone else.
For example if somehow a Mario game was cancelled, I would not even remotely suspect it would mean that the whole Mario IP was dead never to be seen again.
I'll also mention that modern gamers are spoiled. When you used to buy games back in the day like say ohhhh... Starflight, it was not unusual to find out (if nothing else from comments on the box or folding cardboard sleeve) that the game might have taken 5 or so years in development. I think Duke Nukem had eight years it was under development. Now that's a lot when your used games being churned out in a year or two, but within my lifetime it's annoying (and surprising) but not totally unheard of. One of the reasons for long development times was in part they each new game had to have a new engine develeped for it, basically ALL the work done. It wasn't like today where you have all the hard work done a handfull of times as the "Unreal" engine or "Havoc Physics" or whatever. One thing that I thought of early on was that DNF was going to try and pioneer a new engine to keep pace with the other big ones, with DNF itself being a sort of tech demo for the framework (sort of like what the Unreal games themselves have become, intended mostly to demo the Unreal engine for other games). I imagine it was a spectacular failure, but that does not mean that we won't see someone take the Duke character and "world" and develop a new game using an established engine.
