Trying to second guess the collector car market is like trying to out guess the stock market. Some cars will always have high, and increasing, value while others go in and out of favor. It's probably healthiest to just think of your collector car as a hole in the driveway into which you pour money. If you're fortunate enough to sell it for more than you have in it at some point, good for you. If not, just reflect on the pleasure it's given you along the way. In addition to my Z, I have owned a 1970 Porsche 914-6 for 23 years and a 1963 supercharge Studebaker Avanti for 40 years. Both are "rare" cars with production of just a few thousand each. The 914-6 is riding high currently with examples similar to mine selling more than twice what I paid for it. The value of the Avanti has barely kept up with inflation. I've put more money into my Z than I could ever hope to recoup. I enjoy them all and have no plans to sell any time soon.
Dennis