At least _try_ to understand what you are talking about before you criticize. Your rant demonstrates your complete lack of understanding on the topic.
The, apparently, "hidden" knowledge is that Unix shells are designed to process text. And so you serialize complex data structures over FIFOs from child to parent. That can be as simple as comma/colon/whatever delimited bytestreams chopped up with awk or as complex as JSON and parsed with jq.
It’s generally considered best practice (in the functional programming community at least) to write “pure” functions (that is, functions without side effects) because it’s much easier to reason about what they are doing.
So good news, subshells can _only_ be pure (the only way to get anything back from them is if they write some string to stdout), but sometimes you do actually want to have some side-effects (imagine a function that reads a config file and then wants to set some of the global variables to the values found there).
Well, bad luck. If you use the subshell syntax you literally can’t do that.
Think of grep as a way to control flow. If one side effect happens, send to stdout one thing. Another side effect, send something else. Then iterate over those outputs.