I am speculating it improved itself for the 1986 US Open because of the before/after rankings of the course. That's how Pebble got to #1 a few years ago: it improved itself in conditioning. The ranking speaks for itself.
Mark:
The improvement was simply that Shinnecock showed it could host a U.S. Open. That was a quantum leap for many raters.
If Prairie Dunes or Crystal Downs or Pacific Dunes hosted a U.S. Open and it came off swimmingly, they'd move up to the top 6 or 8, too. The ability to host a tournament is seen as Important by a significant sub-set of raters. That's why Cypress Point and National are on shakier ground than the others [and precisely why they were further down the rankings 25 years ago].
No. You are wrong. Ability to host an Open is not in the criteria.
Some people might think, "Oh, 'memorability': having a US Open will help me remember the holes a lot better." Wrong: memorability is specifically defined as design features (tees, fairways, greens, hazards, vegetation and terrain) that provide individuality to each hole.
Other people might think, " 'Resistance to Scoring': surely a US Open makes things harder." Wrong: the proper measurement is for scratch golfers playing the back tees. Not +8s playing special tees. Now, it is true: a US Open course as prepared is hard / harder for everyone. But that's because the course is changed for everyone (including the prototypical 0 handicap rater). It is also true a US Open course is better conditioned -- but the criteria capture that as a first-order effect: conditioning. It's not some magic pixie dust sprinkled on the course by floggers.
I'll go a step further: hosting a US Open may hurt a course's ranking because, in making the course tougher but still fair for +8s the course may be made tougher but
less fair for scratch golfers. So it's not a given the US Open will even help. Hosting a US Open can help the stewards of the course understand, appreciate and ultimately improve the course through things like improving toughness without sacrificing fairness, improving conditioning and maybe even improving the shot values. But any program of improvement can do that.
The US Open, as a fact / criterion unto itself, has zero input into rater assessments. It's simply not on the form. Any rater who somehow changes his scoring simply because the course hosted a US Open is in direct violation of the criteria. Because all raters are thoroughly educated on application of the criteria and evaluate properly, that does not happen.
And Cypress Point and National are not on shakier ground because they haven't held US Opens. They're on shakier ground because their architecture is not strong enough to host a US Open: both score
more than a point lower on RS than Oakmont, Pine Valley, and ANGC.
As I&B continue to improve, their greatness will continue to diminish unless they decide to improve their course to keep up with the modern golfer.