This, in my inexperienced view, is the most compelling reason for Fairphone to try and achieve a FP2.1 (or FP2U etc). Allow people to replace their FP2 batteries, screens as needed with normal wear and tear.
If they want to upgrade to FP2U; buy a new FP2U mainboard module (for compatibility reasons, maybe camera, top and bottom modules would all have to be FP2U) while re-using the battery, screen, case.
From what I can see, suggestions along those lines have been made for a few years on this forum, so I imagine Fairphone HQ have had similar discussions internally, with more detail and expertise.
I’ve searched through the product blog posts on the Fairphone website and I don’t see any posts exploring the pros and cons of these options. Hopefully a bit more detail will be forthcoming in future; it seems likely that a decision has already been made based on the length of time since it was first proposed.