First of all, everyone joked about "little Kenny's joke of a serve". He was a natural lefty who was forced to play righty, which was probably why he was so strong on the backhand side. But you find out how good players are when they face the next generation, and just studying how good Ken still was near 40 tells you a lot. Talking about how he got creamed by Connors isn't fair because he was old and very tired. All of you should reconsider his later career play and those of others while you are watching old Fed and old Nadal.
So we talk about Ken being from the older generation, but he was still winning matches close to 40. Laver was still winning matches close to 40. And Connors at age 39 got far in another slam. If these guys were still competitive at close to ave 40, that's important. We can conclude from that one thing: these ATGs stayed relative for a long time, with Pancho making the QF of RG in 68 at age 40. Nadal could not win it this year at age 35, with much better nutrition and training and modern medicine.
As for topspin, Laver did it from both wings, and he did it with a heavy wooden club.. He had a forearm the size of Popeye's. Borg did not find it easy to beat him on clay. There's a good match on YouTube with Pancho commentating.
Finally, Rosewall's "weak underspin backhand". It was a lot like Connors forehand, almost flat and lethal. He didn't have a strong serve. How do you think he won so much? It was a super precise shot, and every other pro feared it.
Finally: the pros pre-open-era trained in a way that no one else has ever done. They played everywhere, traveled in any way, worked hard for the money. Played on any surface, in any weather. That's why they were so dominant in the early open era. They were tough in a way that the amateurs were not.
You can miss all that because you are brainwashed by what the modern rackets and strings do. Things were slower, and less precise, because that's what you got with that equipment, and canvas shoes, and just a racket press for a racket you could buy for very little money. Give these modern guys those rackets, with those strings, and those shoes, without their teams and eggs and borderline legal quasi-PEDs, without MRIs and micro surgery and you would make very different conclusions.