And yet, if the script is written to POSIX sh standards and has the #!/bin/sh shebang line, none of that breaks. Yes, of course, things do change incompatibly

And yet, if the script is written to POSIX sh standards and has the #!/bin/sh shebang line, none of that breaks. Yes, of course, things do change incompatibly in a decade, probably including some of the nonstandard platform-specific commands called by the script regardless of which dialect of shell. But people who use a nonstandard shell dialect - however common it might be at the time of writing - should be prepared for that shell to become uncommon, absent by default, installed at a different path, incompatibly changed, or otherwise unusable a decade later to an extent that people writing to the standard do not.