That was me (although, I know many others have done it), and yes, it is not the sole reason, but it compounded on top of other issues, some of which were beyond the school's control, and many that were not, that are still plaguing some UND athletics, especially football.
But you are right, at some point, all others things equal (which they aren't, but...) 4 years won't matter. It's like having 4 more years of experience than a co-worker at a job - the first few the difference in experience is going to be very significant, but after a while, they aren't. How long is that while? Based on my own personal job experience, I'd say 10-15 years. Were probably just settling into the point now where the XDSU's are not any further ahead than UXD's due to the sole reason the XDSU's did it 4 years earlier.