It's almost a certainty that all of the drawings, plans, and related engineering information for the Stinger missile still exists. In that sense, the backup is fine. It works. Raytheon has the plans for the Stinger missile and likely much, if not all, information on the design of the original production lines.
But that's probably where the issue lies. Those plans make certain assumptions based on the technology, tooling, and processes in use at the time. The bill of materials may reference off-the-shelf parts that were readily available back in the 70s and 80s but are now considered obsolete. Some of the raw materials specified may no longer in use due to new environmental regulations, changes in industry standards, limited availability nowadays for any number of reasons, etc. Machining processes may be documented in the context of machines, tools, and procedures common a the time that we simply no longer use. Similar challenges exist for every part of the process, from sourcing materials to quality control.
You can document everything with a great deal of detail at the time--likely the case here, by the nature of manufacturing complex products--but that doesn't mean it will be understood in exactly the same manner forty years later. No engineer can predict how processes and standards will change over that kind of time frame; the best they can do is document everything clearly enough that future engineers can work through what's left behind and adapt it as needed in the future.
Bringing in retirees likely expedites that process. They can almost certainly work through it, but if a few phone calls and meetings with retired staff can speed things along, there's no reason not to. It doesn't mean someone screwed the pooch in the past. In a military context, existing stockpiles are meant to give you sufficient time to restart production.
With a computer backup, you can monitor that the system is functioning and verify the recovery process. You can even confirm it directly, restoring backups to a separate system. You can't do that with a manufacturing line that hasn't manufactured anything in forty years. Either you keep it active, manufacturing items you neither need nor want, or you try to constantly update your plans every few years on the off-chance that you'll want to build more outdated Stingers in the future.
That would absurdly wasteful, and even if you catch some changes in procedure/systems/materials, you'll miss plenty of others unless you actually try to restart production. No matter what you do, there's going to have to be a certain degree of interpretation work that's needed.