So you want it to compile to POSIX sh but you're unhappy it doesn't use more POSIX-incompatible features? Maybe full POSIX sh compat is WIP, or in some simpler cases maybe it already is?
As I've just updated my comment to reflect, TIL that parameter substitution is supported (with some exceptions) by `sh`.
More broadly though, yes, there's a tendency for people to pop sub-shells without giving it a second thought. Those aren't free; use shell built-ins whenever possible!
