Question about long-term support for legacy board designs
Handling component obsolescence for 30 years is a difficult task.  Even 10 years is amazing for anything but the most simple of products.  

Simply put, suppliers and manufacturers are unreliable over a 30 year span. As mentioned before, they will stop making chips without notice or simply cease to exist. There was one manufacturer about 6 years ago that had a fire at one of their older factories. Rather than rebuild and start making those chips again, they simply obsoleted all the chips that were made there.  

Despite everyone's best intentions, parts become unavailable.  

If a PCB respin is not allowed, then the only reliable solution is to stock a 30 year supply of all parts. This is more complex than it sounds because some parts have a limited shelf life.  Parts would need to be stored long-term in temperature and humidity controlled storage, and even then some parts will still not last 30 years.  Storing components for 30+ years will require the consultation of the manufacturers to determine the ideal storage conditions.  Documentation on how to properly store and solder the parts would need to be archived.  For example, if a bake is required before assembly then those instructions would need to be saved.

If a respin is allowed, then the ideal solution is to archive all design data and write additional documentation to facilitate redesigning every 4 to 10 years.  In my opinion this is the best solution (assuming a respin is allowed).  It allows the product to evolve over time to accommodate new technology and new manufacturing processes.  This would either lower the cost of the product or give the new versions new features and capabilities.