People keep inventing new programming languages. I'm surprised by how many brand-new languages are adopted by more than just their creators, despite the network effects that would seem to discourage such adoption. Good! Innovation and progress in programming languages depend on such adoption. However, let's not forget that fragmentation of programming languages reduces the sum of those beneficial network effects.
One example is library ecosystems. Every new language needs a set of libraries for commonly used functionality. Some of those libraries can be bindings to existing libraries in other languages, but it's common for new languages to trigger reimplementation of, e.g., container data structures, HTTP clients, and random number generators. If the new language did not exist, that effort could have been spent on improving existing libraries or some other useful endeavour.
Another example is community support. Every new language needs an online community (IRC, StackOverflow, etc) for developers to help one another with questions. Fragmenting users across communities makes it harder for people to find answers.
Obviously the efforts needed to implement and maintain languages and runtimes themselves represents a cost, since focusing efforts on a smaller number of languages would normally mean better results.
I understand the appeal of creating new programming languages from scratch; like other green-field development, the lure of freedom from other people's decisions is hard to resist. I understand that people's time is their own to spend. However, I hope people consider carefully the social costs of creating a new programming language especially if it becomes popular, and understand that in some cases creating a popular new language could actually be irresponsible.