Welcome to the community forum.
Well ok, the Fairphone 1 is old and I didn’t have it and so I have no clue what the cover situation was when it was actively sold.
Edit: So I shamelessly grab a quote from the future 
…
So second-hand could be an option for an ever more dwindling user base.
But what about the Fairphone 2?
Cases for the Fairphone 2 are still available in 4 colours in the shop. That’s 4 more cases than are currently available for the Fairphone 3.
Not counting protective covers from other vendors than Fairphone.
This has been discussed at length in this forum already, feel free to use the search function.
Bottom line is … Even if you would make the hardware parts compatible, state of the industry currently would make the software incompatible either from the start, or at least rather sooner than later. It just doesn’t work that way for the foreseeable future.
It’s inconvenient, agreed, but they are operating on different software platforms which don’t share the accounts.
And this forum furthermore is a special case in the Fairphone online presence, because it is not an official Fairphone communication channel, but really a community forum …