Nobody had all the answers for these three players at their peak, they can won any match they wanted. But the fact is that when Hoad faced Rosewall in big matches, he really couldn't win: five straight losses in Pro Major finals, and I'm sorry but I refused to consider Hoad a spent force in 1961-62, since he destroyed Laver 8-0 in 1963.
This is only a theory.
You are saying: "playing against Pros would have raised their level", and I accept it. This may be true (not sure at all anyway, just look at Ashley Cooper).
But there is a fact: they didn't play against Pros, so their level didn't raise.
Sure, it is a drop, but not a drop in a gigantic black hole such as Emerson's fall after 1967
I think my opinion is more realistic: Emerson level in 1967 and 1968 was pretty much the same, but in 1968 he faced really strong opponents.
Ok, change it in: 1959 was his only dominant year, while in 1958 he didn't win important titles.
Improved by being beaten 18 more times by an injured opponent. I'm impressed. Anyway, I'm not trying to delegitimate Pancho's 1958 title, since nobody ordered Hoad to fall injured.
Please take in account that their head-to-head (wiki says 107-75, which I think is not exact, but it give us an impression) is distorted by the fact that their meet each other 86 times in 1957, with Gonzales at his absolute peak, while they didn't meet each other at all in 1962-63.
Newk was able to exploit them, but he faced Rosewall only since 1968, when Ken was 33, well past his prime. Moreover, Newk was TEN YEARS younger, and still Rosewall defeated him in a lot of big matches: USO '70, Wim '74, USO '74 (add Dallas WCT Finals '71 to the list if you want).
I have to remind you that anyone loses sometimes: Gonzales has also great defeats against Sedgman, Segura, Rosewall, Trabert... why don't you complain about his weaknesses then?
If the Pro circuit was weak in the early 60s, as you said, why should I consider big challanges the first-round guys in Slam tournaments?