editcontentmodel is often needed for debugging, which is one of the main reasons staff rights are handed out, and is not particularly powerful compared to all the other things already granted to that user group. It should be added.
There's also T85847: Grant editcontentmodel right to all logged in users which argues that everyone should have it, which sounds sane but also like it might require lots of consensus-building first. Granting it to staff for now shouldn't be controversial.
I think James needs to have something of a final say but I don't see any problems with adding this to staff. Adding to all users might take more work for security reasons as Tgr points out.