I didn't say there is no logic in much of you've said, on the contrary, your explanation why other coins fail is interesting. I only think you underestimate the inertia of Bitcoin fundamentals, it *may* fork in a few decades if SHA256 is undermined, and it may switch to some new hash function but everything else will stay the same.

Also, you are underestimating the new fundamental advantage of Primecoin: it will be very hard to implement it in GPU to be faster than CPU, unlike other coins. The advantage to mine with only CPU, gives many people a chance to mine, taken away from them by specialized GPU mining monsters.