I stopped caring about POSIX shell when I ported the last bit of software off HP-UX, Sun OS, and AIX at work. All compute nodes have been running Linux for a good long while now.
What good is trading away the benefits of bash extensions just to run the script on a homogeneous cluster anyways?
The only remotely relevant alternative operating systems all have the ability to install a modern distribution of bash. Leave POSIX shell in the 1980s where it belongs.