Electronic – Design for long term reliability

I need to design a small board that will go into a large piece of public infrastructure intended to last many decades. I am looking for papers and the like that give guidance on such design based on real research.

This board will be much bigger for mechanical reasons that it needs to be for even a spacious circuit to achieve the function with discrete parts. Things like wide traces is a no-brainer.

The customer wants to minimize total parts, and wants them to be thru hole. I see the point about minimizing parts, but which parts also matters a lot, and being able to get replacements in the future is important. This function can be implemented with a handful of discrete transistors and resistors, but the customer would rather use a single logic IC in DIP package. He thinks thru hole is more reliable, but I think I remember seeing a study that says the opposite. Also, I'm worried about availability of a 16 or 20 pin DIP logic chip in 20-50 years. But, are SOT-23 transistors and 0805 resistors a better bet? There will be some opto-isolators. It seems to me those will swamp everything else in terms of reliability and future availability. Yes, I'll run the LEDs at a small fraction of the rating to increase life.

So, I'm looking for real definitive research-based information on designing for long-term reliability. This is an area where it's easy to think about the 10% problem but miss the 90% problem that makes the 10% issue irrelevant.

Added:

I'm looking for evidence-based answers. I like to think I know electronics pretty well, and can come up with various plausible-sounding reasons why one approach may be better than another, and I'm sure others can too. However, I don't trust those because what sounds plausible and is based on sound physics may be correct but missing some other more dominant affect. I'm worried that this is where educated guessing could lead to significantly wrong conclusions. That's why I'm asking for evidence-based answers, papers from actual studies, rules NASA might insist on, etc.

Added 2:

Consider the environment "industrial". I'm not sure how well the environment is controlled if at all. The boards will be protected from the elements, but possibly no air conditioning or heating. I don't know about vibration, probably not much.

These boards will be installed in a cabinet that houses other parts of the electrical system. Service technicians can walk up to the cabinet when necessary. Difficulty of servicing isn't the issue, but downtime is. This is not what's going on, but imagine if a interstate highway was shut down until the system is up and running again. Of course there is redundancy already, but failure is something you really want to avoid.