Frequent users of Model Builder will have experienced toolboxes and models that become corrupt and cannot be opened. This is a very serious problem and can result in a great loss of work or possibly the permanent loss of valuable intellectual property.
As has been suggested for MXD files (see https://community.esri.com/ideas/2884), an open XML format for toolboxes (TBX files) would greatly ease difficulties users experience. This would not only allow (experienced and cautious) users to fix their own toolboxes, but also would allow ESRI technicians to investigate corrupt toolboxes more easily. It would also make it possible for the user developer community to build repair tools.