Everyone uses USNWR rankings... except for university deans, provosts, chancellors and presidents.
So it's relevant unless we're talking about a criteria on which a university will base a decision (i.e., realignment).
DU is a good school with a solid reputation regionally but it doesn't matter what USNWR says, it has nowhere close to the academic prestige that CU has.
Very simply put DU has no place close to the research resume that CU does and for better or worse the academic world values research.
CU has the Nobel prize winners on staff. It also has a strong record nationally in terms of Fullbright Scholarships, Carnegie Scholarships, etc.
Sadly by many measures CU-Boulder has seen some declines in terms of academic standard and reputation in the time that Phil D has been Chancellor but locally, nationally, and internationally CU has a stronger reputation than DU in most fields for which they have shared programs
More significantly college sports is about the money, plain and simple.
In looking at potential candidates for expansion conferences are going to ask the same question, how much financial value does this school add to our conference.
This can mean looking at factors such as what kind of ratings does a school draw on TV games, nationally or regionally? How well do they draw both at home and on the road? Does the school attract major advertisers? etc.
The B1G and SEC are in a position that they can dictate to potential expansion candidates the addition of varsity sports. The B12 isn't.
The B12 like the PAC12 is worried about surviving as a conference at the highest level, they need schools that add value in revenue sports.
The B1G has the money. They can tell prospective candidates that they will be receiving enough extra money to pay for an expanded athletic program, take it or leave it.