You simply cannot compare drivers from completely different eras to an objective standard. We can each argue the toss about where one era ends and another era begins but the simple fact is that each rough era/decade/period/whatever is its own crucible wherein you can only reasonably compare the drivers from said same era.
It's a bit of fun to say "What if Senna drove today, what if Schumacher drove back then" etc. but you can't extrapolate a comparative standard from that. Too many things are different; when Jim Clark was racing, tyres were expected to last 3-4 races; when Fangio was racing, you could abuse the technical rules to have a closed-wheeler; when Senna was racing, cars were still expected to have a 50% chance of mechanical failure at any given Grand Prix; hell, even when Schumacher was racing, there was refuelling to add a strategical problem current and other drivers don't and didn't have to worry about.
I've always found the athleticism argument interesting though. Obviously, drivers today are supreme athletes and have to maintain a rigorous regime to stay in that shape but that doesn't mean drivers from other decades weren't fit. Athleticism and fitness are two different things. I'm sure when Nuvolari and Caracciola where struggling to turn the wheels of their power steeringless, heavy, no aero cars it was a bloody tough endeavour to stay motivated and strong enough for 90 laps.
What you might be able to do is have each of us list who we think are the best 5-10 drivers ever (not necessarily our Rubens Barrichello favourite) and aggregate the results to find an average frequency. And even that has the obvious pinch of salt that some people might only rate drivers they've seen as fans in real-time and others might try and rate drivers they've heard about who raced previously; even at the age of 28 I cannot rate Senna or Prost as drivers whom I actually saw race.