I kinda think that if AdSense were to "belly up" sometime down the track there'd still be other ways to make good money from well tuned site (or sites). Having said that I think that AdSense or something similar will be around for a while. After all it works rather well. You're looking for something in particular and there are links to help you find what you want - be it merchandise or information. If it works, it'll most likely stay.
I agree with you, why should it die when its making everyone money.. if it wasnt making advertisers money they wouldnt be paying us.
The question shouldn't be about the life of adsense, but about how long adsense will be valuable. The amount of spam sites using adsense will increase, the amount of click fraud will probably increase, all of these things will add a cost to operate adsense. Google wont bite the bullet. Yet on the other hand, google may decrease the spams and click fraud and the value of adsense increases. Who knows...
You are being pessimistic It is like saying, due to the theft that happens in supermarkets/shops, shop owners won't sell anything, thus collapsing the whole economic system. True it might diminish its value but it will never ruin it.