One interesting thing that Gardiner Bryant (The Linux Gamer) reported from talking to Purism was that “the Librem 5 is a platform and there will be the kind of compatibility that you would expect from a platform.”
That comment gives me some hope that Purism is designing the Librem 5 like the SHIFT5.X phones so that parts from the SHIFT5.3 can be used in the SHIFT5.1, which makes upgrading possible. I would love to have a phone where the motherboard with the CPU, RAM and SSD can be upgraded and all the other parts can be reused, so I really hope that is the goal at Purism.
At any rate, the ability to change the cellular baseband means that you won’t have to junk the phone just because you moved or changed cellular provider and need to use different bands.
I also expect that the Librem 5 will have excellent resale value, so that if you decide to stop using it, there will be buyers who will give it a second life. Not everyone demands the latest CPU in their phones.