The whole thing boils down to the time value of money. Assuming you had $100 today, it would be worth less 1 year from now because of inflation, plus you would be missing out on the investment return you would have earned.
If we look at some of those old guitars, a similar logic applies.
I bought a Jeff Hanneman LTD Eclipse for about $800 in 2013. When I sold it a couple of years ago, I earned a couple of hundred dollars more than I paid for it. So the investment panned out. It'll also pan out for the buyer, because as the guitar becomes harder to find, there will be some people willing to pay a premium for a Jeff Hanneman signature guitar.
Oh, and it was MIK, but superb to play. I really liked it and only moved it to finance something else.
I have a couple of ESPs now, and I have no doubt that if I ever try to move them, I'll make money. But my Warwick Vampyre bass, on the other hand, is hard to move.
In that regard, sometimes situations occur that increase the desirability of things like guitars. Could be the death of an artist, or rules on certain species of trees. That's a good time to cash out, if you want/need to.