There is no "better" in any other coin
I think you just proved my point here.
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.
If Bitcoin had to change the PoW, that would probably end up killing it.
By the time a reason comes to change the PoW, the amount of money invested in double SHA256 silicon would be much greater than it is today, and changing it would alienate the current mining community due to how expensive it would be to actually mine.