Pretty much every record in every sport is arbitrary, yet people like to set goals for motivation.
This doesn't seem necessarily true to me. If Magnus was in his prime in say 1980s it's not clear if he could ever get so close to 2900 since every other super-GM had lower ELO which would make his progress slower than now. Not only do we not know how an average 2022-super-GM compares to a 1980-super-GM, we also don't know how Magnus compares to an 1980-super-GM. E.g. Tal achieved his peak ELO 2705 in 1980, but at that time Korchnoi had an ELO of 2695 (diff of 10), whereas, Magnus currently has 2864 and #2 Ding Liren has 2808 (diff of 56). But Magnus' peak rating of 2872 is 30 less than Caruana's peak rating of 2842 during the same time.
I suspect this is false. Chess is one of the few endeavors where we have decent historical records. Combine this with the fact that computers can now analyze lines a very long way, and I suspect that you can do things like compare how often GMs missed a superior line of play.
By most measures, the GMs of the past played weaker chess than our current GMs. The current GMs benefit from having started at younger ages with a larger body of chess theory and super-strong computers to work with and against. I suspect that the GMs of the past would be as strong as the GMs of the present given those. However, they didn't have those and, consequently, the chess, itself, was weaker.
Side note: I adore Mikhail Tal's "This position looks interesting and complex. Banzai!" games. However, most of them would get him absolutely creamed against the current top players. And to be fair--even the top players of his own time tended to refute them. However, Tal's games are often way more exciting than everybody else's.
It's still impressive as heck.
One tempting comparison is Usain Bolt, but it feels a little different to me because he dominates both objectively (WR) and versus his competitors.
The decathlon works the same way- the points scoring there isn't relative like Elo is, it's done by performance in an individual event. You don't get any extra points for winning a given decathlon event, there's a scoring scale for your marks regardless of position.
The exact same performance will differ in how it affects two players' Elo depending on both players ratings. If all players' ratings were to be 100 points higher with all else equal, the top players ratings would rise as well. If everyone in my Mile runs 10 seconds faster, my time doesn't also get 10 seconds faster.
Basically, one record is an individual performance mark. The other is a ranking, which is a different kind of metric entirely.
With all that in mind, chasing a world record Elo is still a worthy goal IMO. The competitive chess scene is old and robust enough that it remains a meaningful benchmark, even if it is a relative measure.
I wonder if it is a valid strategy if he drops out of World Championship title contests to allow others to increase their score by not losing against him. I would have thought that wouldn't move the needle much but I guess he needs everything there is to get to 2900.
There are going to be team compositions that didn't win an NBA championship that are stronger than ones that did win a team composition simply because at the time the competition was stronger and so they weren't the best. So, if you use #ofChampionships to generate a ranking of all team compositions there's going to be a lot of debate that the teams aren't actually ranked by strength since some of the teams that racked up championships did so when other teams sucked (I know less about NBA but for NHL's original 6 this becomes the case, the competition was so weak that some people didn't know they were drafted because it wasn't something you cared about).
To get back to the original point, your ELO rating is in relative to your peers. The idea is that for a given point differential you have a x% chance of beating them. So the point being made is that you can't use ELO to compare anything besides two players who can physically can play each other as taking a playing in the past with n elo and having them play a current player with n elo won't match the x% chance.
To counter some of the other points (bowling and the mile). These are precise rankings not relative. If somebody runs a 4min mile in the past you can expect them to run a 4min mile in the future. This doesn't mean that they'll win the same %of races though as they did which is what elo is more about. Same thing with bowling, if somebody constantly bowled a 280 in the past you can expect them (ignoring lane/oil changes) to bowl 280 in the future but if the competition now bowls a 290 that player is going to suck instead of being a legend. So the bowler of the past may have had a very high ELO but now when taken into the future will (after many losses) have a lower ELO despite having the same peformance.
Federer getting grand slams - all against local in time competitors - is impressive. Had he played 50 years ago, he'd have likely gotten more. Had he played in 50 years, likely less, expecting competition to improve.
Same for NBA records - all against a moving, relative backdrop.
Same for wrestling, baseball, hockey, and on and on and on.
Just because your score is relative to others does not make reaching some milestone, especially this one, less impressive.
Also, it's easier to have a higher ELO if others do too, not the other way around.