It's because its all open source. Anybody can make any changes to a client that they want. If the company was wasted, then some other group (or the same group under a different name) can continue the effort. Even if a switch was added that could deactivate a client (I believe XoloX tried that, but it didn't work right) somebody would use the open source would be used to remove it and make it public. Simple.

On the other hand, closed source projects that have a death switch or forced upgrades can quite simply be taken out. Judge orders the switch thrown or a forced upgrgade to a useless version and *bam*. Napster's network system was as open as Gnutella (without the changability though), but the closed source app was their demise. The same will most likely be true with KaZaA and it's FastTrack network.

Although not all Gnutella clients are open source, enough of them are to make attacking Gnutella relatively worthless. Though don't think that the RIAA isn't stupid enough to not try it . . .