It is hard to predict what will become a collector’s item, but a bad economy which limits the number being produced can help make something a collector’s item. For example, only 50,000 of the IBM Simon were ever produced, so it is now very valuable today, whereas over 6 million of the original iPhone were sold, so it isn’t very valuable today.
The Nokia N900 was considered a failure when it was released, but it has developed a passionate community of users and the phones are still valuable today. However, I suspect that most people currently using the N900 will abandon it, once Maemo Leste is ported to the PinePhone (especially if someone makes a physical keyboard mod for the PinePhone).
The reason why I predict that the Librem 5 will become a collectors item is because it will have 1. limited production so rare, 2. a passionate community of users who want it, 3. physical features that few phones will have, 4. a distinctive appearance (thickness and kill switches) that make it stand out, and 5. longevity (in terms of software support and ability to switch out the cellular modem and Wi-Fi/Bluetooth).
Collectors items are more valuable if they can still be used in the present day (which is why people are so passionate about old cars because you can still drive them), so the Librem 5 will be more valuable if you can still use it as a working phone 15-20 years from now.
I predict that a lot of the current FDD-LTE networks will be switched to TDD-LTE networks in the future in order to save frequency space, but that won’t be a problem because you can switch out the M.2 cellular modem in the Librem 5.
Even if it isn’t possible to ever put a 5G modem in the Librem 5, due to the different antennas and power requirements of 5G, I predict that TDD-LTE networks will still exist 20 years from now. There is no good alternative to LTE in the offing for long-range networks outside of dense urban areas, so cellular networks will probably continue to use LTE. It is very likely that the LTE standard will last for ~30 years, just like the 2G standard has, because 5G and future standards will probably be designed to support existing LTE standards.