...Honestly, unless you're 70 or older, you don't really know how the '50's were, you just have 2nd hand nostalgia. I think the 'weak' men of 'today' are really just a few individuals you know of with low self esteem and nothing more.
I'm going to have to go with this, I think.
Really, if we were to actually to get to the bottom of this, we'd have to first delineate just what specific characteristics we're talking about when we say "manly." Having accomplished that, we'd then have to somehow look at just how well your average men from decades past actually fit that bill. Sure, you watch some John Wayne, and maybe you think that was some sort of accurate cross-section of your average male at the time. But I'd wonder just how accurate that assumption would be.
Sure, there's plenty of men out there that don't live up to classical standards of what everyone could likely agree would constitute a "real man." (Responsibility, integrity, providing for your family - however that manifests, standing up for yourself and others, etc.) But I'd wager that's always been the case. There were probably plenty of cavemen, even, who would likely fit that bill of being "less than a man." As far as what percentage of men today actually qualify as "men," compared to past generations - that's something that would have to rely on something more than simply anecdotal evidence.