If you compare and look at a collection as merely from a valuation point, you are in my mind no longer a collector but a speculator. To own something to hope that it increases exponentially in the future is just that a speculation. I think that military collecting is much in common with persons who are into vintage or special vehicles. Typically, these cars are very old, expensive, difficult to track down especially originals. You will put your time and effort meticulously restoring, intimitly curating and observing every little facet of the item. The detail, the love, the time spent tinkering. (Now with exception to folks here who have really become masters at restoring Haubes) I strongly recommend not "tinkering" with your items.

Now, to go back to that car enthusist statement. If you can go to a local car club or find a vehicle that has that loved and brought back to a showroom condition time capsule car. The value of that car is still what a new mid-range car is worth now. Here is a lovely specimen.
https://classics.autotrader.com/classic-cars/1967/ford/mustang/102612172
This is, in my mind, a cherry car. Man, that would turn some heads. But a new Pickup truck costs the same. Both are vehicles. Betcha the wife or ex-wife believed her husband when he said this vehicle was worth say a hundred thousand dollars. That the time, effort, in say, tracking down a small detailed piece. That individual piece was probably quite expensive on it's own. Now, when it is time to liquidate due to divorce, death, or other tragity...This is the price where it is, and where it still sits after over 30 days....

This person never thought that it would have been on the market for this long. This is a great car, Damn near mint. This was this persons "pride and joy". But you know what? If it was a death, it still was this persons pride and joy. It gave them a great hobby and a great item to pour time and attention. It was their passion project. For that, I smile and reflect on my own passion projects. It might not make sense to anyone else, they might not see the item except for the "supposed" resale value. But, that's the world of collectables.
A car is still a car though. It can be driven and perform functionally in the current world. A leather haube? not so much.....
So there are lots of things that we might have a bit of remorse for missed opportunities. But, if you collect for what you really like and what is your passion. Then, recouping or even the possibility of making a profit really doesn't enter the conscious thought of that individual.
I am grateful for forums like this where we can share our similar interests and passions together. That is what I am grateful for.
This is just my nonsensical rantings.
Best Regards,
JustinG