I don't have much to add as I am late to this party. Allow me to observe that all ratings are subjective by definition. The theory is that by exposing the courses to a large enough sample of "qualified" raters the quirks contained in each individual's subjective assessment will even out and a consensus position will be reached. It is also obvious that, given the relatively narrow spreads in the numerical ratings, changes within the rankings, particularly at the lower end, can occur without significant controversy.
The bifurcation between "classic" and "modern" courses works as a marketing tool. It also presents an interesting opportunity to evaluate the impact of technology on course design. Which is better? Its really a matter of taste. I would suggest that the classical courses have withstood the judgement of time whereas many of the moderns are simply too new for us to determine where they ultimately will rest. The numerous times that new hyped courses come "out of the box" very high in the rankings only to fall steadily, sometimes out of sight, suggests that some time is needed to determine where a course belongs. Thus the classic courses are more likely to retain their status vis a vis each other and the modern courses should be given some time before we suggest that they supplant those with proven staying power. This could happen, I am merely suggesting that we should not jump the gun.
I agree with Barney that as you go further down the list for both classical and modern courses the quality decreases but I would observe that would be expected as, after all, the ratings are numerical; they are supposed to get worse as you go down the list (or less outstanding for the sensitive). Whether one would deem them mediocre or use a less pejorative term is a matter of taste. Mediocre is not a term I would use for many of the courses I have played on these lists. But I agree that there are numerous courses that could be substituted in the lower tier without any real complaints. There are a lot of very nice golf courses which are just below the cut line..
I won't comment on the superiority of grasses and the like even though I spend a lot of time involved with greenkeeping issues. The courses on these lists rarely have playability issues and I view these arguments as rather weak "make weight".
Finally the last grand question; should my friend Barney play Lawsonia? Frankly, my dear, I don't give a damn! Do what makes you happy. But at least be consistent. On the one hand you ask, what good would it do for golf if you played Lawsonia and in the next post you ask whats in it for you? Those appear to be somewhat contradictory, but lets answer them.
It probably won't make any difference to golf whether you, or I or for that matter any one of us plays Lawsonia or any other course. None of us are that important nor do any of our opinions carry that much weight. As far as what it will do for you, you might have a good time on an interesting course. You might learn something, if you're receptive, but you might not. You have already indicated you don't learn from reading works by the old masters so I can't predict what you'll learn from playing. But I don't know whether you'll gain anything from playing Erin Hills either. Each of us has a finite yet unknown amount of time to spend on this orb. Since there is no moral imperative involved, do what you think will make you happy. But don't elevate your choice to anything more than that. We all spend a lot of time analyzing GCA and we all take it too seriously, myself included in spades. But sometimes we get carried away about its and our importance.
I hope you have a great time and I'll be interested in your reactions. You're always welcome to join me at my place.