Decentralized crypto-currencies could be a one shot deal where only one is the winner and towers over all the others which bounce around at ankle-height. Bitcoin has a humungous potential vulnerability -- the 51% attack. The Bitcoin network can be shut down (i.e., all transactions ignored) by any malicious party with 51% of the hashing capacity.
Bitcoin hasn't had anyone (that we know of) even come close to successfully doing a 51% attack. With multiple ASIC vendors shipping (ASICMiner, shipping to their own mining operatioin) and Avalon ASIC and more coming (BFL, presumably ... have faith ), there's no way a competing effort using FPGA or GPU has a chance to reach 51%. Maybe someone is hard at work in stealth mode building new ASIC hardware with an unlimited budget, with malicious intent. They'll need to beat the free market, and it looks like the free market is winning. It has quite an advantage though. Bitcoin is currently paying out $3.5 million worth each month to miners, and miners are buying millions of dollars worth of ASICs. I can't imagine any single entity that could justify a gamble on procuring ten million USD+ to try to build technology to take on some fledgling digital currency.
But compare that to a fledgling alt currency that is decentralized and uses a similar proof-of-work approach as well. Bitcoin long ago subsidized the CPU miners, then the GPU miners then the FPGA miners using the seigniorage that came from an exploding exchange rate (able to pay out currency worth ten to twenty million dollars in each of 2011 and 2012, and at an even greater pace so far for 2013). Bitcoin is currently probably too big to bust today, but back when it was vulnerable nobody took crypto currencies seriously enough to take a serious stab at killing it. Alts don't have that luxury. They aren't getting the high valuation to build a resilient network of hashing capacity before anyone notices. As long as they stay under the porch, it doesn't matter that they exist. Start to take significant value from the big dog though and it wouldn't be surprising for a couple hundred grand worth of idled GPU equipment to be recommissioned to see just exactly what happens when a functioning and growing decentralized proof-of-work digital currency gets hit with a 51% attack.
Now that doesn't mean it wouldn't be good for one or more alts to succeed (we all benefit when healthy competition exists), or that I don't want an alt or three to succeed (I've at various times held some alts), it is just that people do things to protect that which is valuable. Most alts work like Bitcoin where any peer node can introduce a solved block. Or it can be done through Tor as well. So a 51% attack can be done without anyone knowing who the attacker is. Add these up and you get a near certainty that someone would start thinking about doing an attack on an alt once it starts gaining altitude.