My thought process is that in order for altcoins to have longevity, their value propositions must be larger than the value that BTC has from the network effect. Currently, many altcoins site that their faster block rates give them a higher value proposition but once third party handlers get involved to speed up transactions to credit card-like speeds, the block rate won't give them much of an edge. The proof-of-work authentication model that BTC uses is costly (energy and hardware wise) but I have yet to see an alternative cryptocurrency (such as Ripple) implement a better system.
It makes sense that LTC/PPC have value currently because the faster transaction times actually gives them value but once third party transaction companies get involved to implement a PayPal-like system to speed up transactions, LTC and PPC will more than likely fall due to lack of innovation.