Daniel Krawisz has taken the time to do the cryptocurrency community a service and come down systematically, hard and at times hilariously on the many weak arguments in favor of various altcoins. In the process, his article reveals Bitcoin itself to be even stronger than it is often presented. He argues that the actual alleged threats to Bitcoin from the likes of double-spend attacks, 51% attacks and mining centralization are each much less realistic and significant in the real world than they are sometimes made out to be (although mining centralization is still something to keep an eye on).
His displayed combination of solid technical knowledge of the field with consistently sound economic reasoning is refreshing and valuable. This is more than a thorough critique of altcoins. It also functions as a fresh defense of Bitcoin against a number of typical technical and economic objections and concerns.
I had previously come to the working impression that if altcoins did gain any useful function, it would have to be in a niche application, or would simply be as a research platform to feed into Bitcoin development. Reading this article reinforced that view, and even suggests perhaps taking it further.
So go and read The Problem with Altcoins.