"??" isn't recognized in php 5.5.*

Happy to support Drupal 7 this year but can't support insecure and outdated version of php.

I strongly disagree with the statement in #4.
with the current operation of updates for drupal 7 if you don't support outdated versions of php the site immediately becomes non-functional; there is no warning about this. In addition, the cost of maintaining compatibility with older versions of php is almost zero if not negligible.
Unfortunately, in some business realities it is not possible to easily update php so an alternative action is necessary, which, I repeat, has no particular constraints for those who develop modules in drupal.