Speaking of counter-intuitive, it turns out that tail enders are contributing little, if anything more in comparison to their teammates than they were 80 years ago, something which I was surprised to hear so I had to go and make this chart:
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Makes one wonder if the apparent greater scoring ease of modern tail-enders is simply a reflection on batting becoming easier in general.
Why is it counter intuitive to see tail-enders contributing the same number and proportion of runs compared with their counterparts 80 years ago? Most tail-enders have always been selected for their bowling ability, so I see no reason why current bowlers would be more talented than those of yesteryear at a secondary skill set on which their selection is not based.
I guess these days batsmen who can keep are preferred over pure wicket keepers, which would reduce the length of the tail and push the pure bowlers slightly further down the order than if someone like Strudwick or Duckworth were keeping. But most modern keepers would bat above 8 in the order and this selection policy would have no impact on the bowlers selected, so the impact on runs scored at positions 8-11 would be minimal. You're possibly just replacing an old keeper with a modern bowler, neither of which were selected for their batting.
One could also argue bowlers these days may in general spend more time working in the nets on improving their batting than those of yesteryear. But if you don't have ability to start with, the potential upside of hard work and training is minimal when faced with Test standard bowlers who one can barely see or against whom one doesn't have time to react to. Jimmy Anderson is a case in point. He is known to have worked very hard on his batting over the years, but his output has improved very little because against Test bowlers he simply doesn't have it in him.
If batting has become easier in general wouldn't you expect the average of tail-enders to increase and their proportion of runs to remain constant? Only the latter is true. Perhaps tail-enders appear to have greater ease in scoring because the introduction of T20s has inspired them to adopt a more attacking approach to batting. But a more attacking approach merely increases the speed of scoring, not necessarily their total runs scored. If the batsmen are taking more risks and getting out more quickly, the number of runs they score, which is what you have measured here, may remain relatively constant.
I know your expectations are consistent with the views of contemporary cricket commentators and journalists. But if you listen extensively to old commentary or read old articles from the 1960s, 1970s and 1980s, they said the exact same thing then. It wasn't true then and and it isn't true now. A great cricketing myth.