Alts can get wetware to divulge private keys...proved
[1] The first/largest of new class's if alts will likely survive, the rest not to any significant degree. So 1 or 2 major hash coins, 1 or 2 POS/HYBRID, 1 or 2 merge mines, 1 or 2 consensus eg.
HASHERS BTC
POS/HYBRID PPC
Consensus Ripple
Mastercoin,
Doge (first viral meme coin)
NMC merge mine, maybe IOcoin.
Conceptually I divide coins in to classes then alts of that class.
[2] Every processor/thing that makes heat runs and alt and several chaf alts in case you use your hash to attack them, thus opening you to attacks as you have diverted your hash, thus your alts face forking for attacking as you become a weak target. A sort of mutually assured destruction comes into play. But POS proves more efficient so displaces them.
Alt-gnostic App
[3] The app that allows any one to add in their alt, such that the APP, will calculate the value market of the alt you are using for the good or service you are purchasing. (ripple esque?)
Market Cap
[4] The market cap of backbone coin is large eg PeerCoin but less than expected due Class coin to alt dilution.
[5] However the diverse nature and swift evolution of alts force majors to keep on their toes
[6] There are only so may good DEV's and DEV teams around these limit the majors as well.
BTC has enough devs following that it can likely adapt to any threat/issue, so it going to be hard to displace. The only threats it face is if the adaption decsion making mechanism become to ossified and so is not dynamic enough to adapt. Also say BTC adopts POS then, it would make PeerCoin alot more likely to be the number 1 or take a much larger part of the market share.