And yes, network effects are so important—your C++ example is spot on. One recent example that comes to mind is the adoption from python2 to python3, showing just how big a barrier those network effects are to adopting something incompatible.
It exposed the slowness issue though. I have a plan for that, but it's a bunch of work.
It will help me if people try it and give feedback -- I'll give details in the post.
I definitely agree about Python 2/3. I think you basically can't overestimate the amount of inertia a popular language has. I'm sure there will be patches floating around for Python 2 for a long time after the core team stops maintaining it next year.
Another example is all the compile-to-JS languages. I think TypeScript is the most popular in part because its transition path is the easiest, i.e. it's a superset of JS.
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Why Create a New Unix Shell? http://www.oilshell.org/blog/2018/01/28.html
I took a very long route to doing it though, first implementing OSH (which will run existing shell scripts, which is close to done, but slow) and then Oil (a new language, not implemented).
This is because POSIX shell/bash has a really strong network effect. zsh and fish are better in many respects, but people are locked into bash because that's what "everybody else uses".
It's analogous to how C has a really strong network effect because e.g. every kernel is written in C. So C++ became a popular language in a large part because it's compatible with C. (Though the analogy breaks down because the Oil language isn't a superset of bash, so we can break free of legacy. The project is though.)
In my mind, I think of Oil as a mix between Python and shell, with some elements of Ruby and JS (blocks, function literals, etc.)