I still find it frustrating that my P65+ won't fit on an XF. Is there an actual technical reason or is it an elitist thing?
The main issue is that while the P65+ is only 10 years old it uses the P/P+ interface/protocol which dates from 1998.
1998 - Protocol for the H/P/P+ interface developed
2008 - P65+ released
2011 - IQ released
2015 - XF released
Could P1 have made the XF support the H/P/P+ protocol? Of course; in programming/development almost anything is possible. But it would have meant resources taking away from work to make the XF better.
P1 has a pretty good track record of forward/backward compatibility. For example the 2018-era IQ3 100mp works on a 2009 DF body and a 1998 LightPhase is still supported in the 2018 release of Capture One 11.2, both for raw processing and for direct tethering. That the XF doesn't support the P+ is an exception to that, but one that I think is pretty reasonable. But I can definitely understand your frustration. Have you tried an IQ series back? In addition to XF compatibility they offer a hugely improved experience over the legacy P+ platform.
The XF interface/protocol is intended to be a foundation/platform for a decade or more of development. You can see the result of this in Feature Update 1,
Feature Update 2,
Feature Update 3, and
Feature Update 4. Each of those updates has had more and better feature improvements than the previous ten years of body development. It's hard to sustain that kind of pace of improvement and creation of really useful tools AND support a legacy protocol from 1998.