There are http://www.commandlinefu.com and the like. I copy and paste stuff from the web into the shell occasionally, I'm sure everyone does. I do use fish, but it is annoying sometimes, it's missing the most basic stuff like backticks. So much so that I think I'm going to stop.

For starters, any utilities based on sourcing won't work, such as nvm/rbenv/etc.

That's technically true for zsh in cases, but it's far easier to port or add compatibility for zsh than fish.

Yes exactly.

Today I learned people actually write / use scripts without specifying the interpreter at the top of the script. This is like writing a Ruby script and expecting it to work when calling it with the Python interpreter.

Anywhere that matters. Besides, still more places than fish or zsh.