I generally agree with the OP's point. Speed and strength need to be applied to a particular discipline like boxing, hockey, football, basketball, baseball etc. In some cases, the greater physical abilities are too much for technique, smarts or whatever you may call it to overcome. I think linemen in football are a good example of this. Modern NFL lines would manhandle 1960's teams.
On the other hand, NFL quarterbacks provide an example of physical abilities taking a back seat to the brain. Drew Brees wouldn't be on an NFL roster if size strength and speed were the only criteria to playing QB. Pure physical ability is clearly less important than intangibles because the best QB's are almost never the most physically gifted. Jeff George probably had the best arm of any QB, but his career was a bust.
The same principle can be applied to other sports:
Hockey: Gretzky wasn't the fastest or strongest player of his time, he had the best vision and anticipation.
Baseball, filled with endless prospects who never panned out because they didn't develop the eye well enough to deal with MLB pitchers.
Basketball: The importance of vision, particularly with point guards.
Boxing is similar in that vision and anticipation of a live opponent dictates the physical actions that are taken to an enormous degree. Much more so than sprinting or weight lifting.
So to me, when you consider an athlete in boxing, the mental factors are so critical that they have to be considered athletic ability.
At similar weights, I don't think the boxers of today have progressed to the point where the older fighters can't physically compete, like NFL linemen have.
As for the likes of Joe Louis or Rocky Marciano against the Klitschko's and other much bigger heavyweights, it's tough to say. I think Louis would fare well, but I'm not certain.