I saw Marichel pitch many times and never saw him throw less than max effort. To the point where he nearly fell over on every pitch. Spann was a location pitcher. I believe Carl Hubell who was at that game called Spann an Ironman
There were plenty of guys that threw very hard that routinely pitched complete games at max effort levels (Ryan, McDowell, Bunning, Carlton, McClain, Seaver, Gibson, Drysdale, Koufax) so that argument doesnt hold
My analogy is that I was a 2 hour a day, 6 day a week weightlifter for 15 years. Never was I able to increase weight and performance unless I increased effort. So when the prevailing attitude is that a pitcher can only throw 90 pitches on a 5-6 day rotation that becomes their limit. They have been babied to the max.
If you had told these guys that they only had to pitch 5 1/2 inning they would have said let me pitch on 3 days rest