IMO compatibility issues are the result of conflating a CLI with a scripting language. If a script was written for bash, run it with bash, not fish or zsh. I can withstand having two shells installed.

You can, but if Fish's value proposition is ease-of-use, then people who can't withstand that need to be able to use fish easily still. Not being able to run `PORT=5000 yarn start` or `make && ./a.out` or whatever snippet you copy-paste from the web without understanding are going to be a big pain point for newcomers to fish.

Don't get me wrong, I'm a happy fish user myself. But breaking backwards compatibility with shells from the past 40 years and then saying "you're doing it wrong" is not how to make easy-to-use software.

Fish 3.0 addresses the common `&&` and `||` operator incompatibility and if you pasted the first snippet it suggests that you should use `env`:

fish: Unsupported use of '='. To run 'yarn' with a modified environment, please use 'env PORT=5000 yarn…'

They fixed many of those recently. And just in case you have a script that doesn't run, you can install a plugin called `bass` that basically runs them in your native bash without leaving fish.