I do not agree with your opinion. Not even close. The easiest example is to watch Efren Reyes in an old match vs Efren Reyes in the 2000's, when modern, standard equipment really had taken root. Night and Day. To new viewers, he looks unbeatable in the 2000's. But he was just as good in the late 80's/early 90's, yet he looks like he struggles in every match when you look at him in those videos. Because he did. He didn't win many tournaments back then. Possibly the greatest cue ball control of any modern professional, and he visibly struggled. It certainly matched my experience. I had my first 3-pack on new Simonis, the first or second time I ever played on it, where I struggled to have a 9-ball break and run on the old, slow cloth in our Student Union once or twice a week. It was so much easier to move the cueball with less effort. That made stroking and pocketing a lot easier.
Ehhh.... I think that a main factor is that the older cloth almost always came with bigger pockets, and as a result, lots of players never were under any real pressure to really work on having a laser straight stroke. And actually... Slower cloth is inherently more friendly to poorer strokes on all the "regular" shots played with english. So much so that players of yesteryear almost always played cut shots with running english, a practice that would not get you very far on fast, slick cloth. Sure, some of these type players still win matches over really good players.. But do they win majors? See: CJ Wiley. He was actually playing some after the move to Diamonds.. He just couldn't adjust to the different style of play. Also, see: Oscar Dominguez. His stroke is just a hair less accurate and smooth than the elites, and as such.. He sometimes stumbles at critical points in matches against elites.
Now.. That being said.. I agree, that running racks is easier on slick cloth, to a certain extent.... When they all lay nice. But when they don't.... Lack of precision in both stroke and general overall approach rears it's head, and the elite players still separate themselves from the pack. I would argue even MORE so than in years past...
Case in point... John Horsfall, a Canadian player, won the Sands Regency at some point in the 90s. Now, while the Sands Regency events had a lot of dead money, and were very much akin to Turning Stone.... I don't think under the more challenging pockets, 9 on the spot break rules, and slick cloth, that such a player has any chance of winning such an event today. And the thing is... EVERYBODY runs more racks as a general rule.. But it doesn't change who ends up winning the events. Because the less accomplished players may run a three pack... And then get caught out when they need to pull out a world class shot with precision position, and they end up turning over the table to their opponent. Who is an elite player and then runs 3 racks, and handles the next critical shot perfectly.
The point being... In the early 90s, I too played on slow cloth Gold Crowns.. And the games did tend to be slower, less runouts (at my level), but then the games were decided by superior safety play and strategy. To even be able to enter tournaments with the monster players of today, those safety and strategic skills are part of the BASIC skillset you must have.. And the accuracy of stroke is what decides if you put down the critical shot to keep momentum going or not. Or whether you execute the safety shot perfectly and freeze your opponent to a ball.. or whether you leave a few inch gap that allows for a jump shot.
I agree with what somebody else said... Slower cloth greatly improves one's margin of error on position shots. You can get thin on a ball, and not have to worry too much about it. Literally being too thick on a ball was the ONLY thing that kept people from running out, in a lot of games. If you struggled to run out on the slow Student Union cloth... You probably consistently got too thick on balls. In the modern game.. If your pattern play is not TIGHT.. Getting too thin on a ball greatly increases chances of "not" getting out, if you are not completely dialed in on 2-3 rail angles and speed being melded at just the right ratio to recover.