Technically, the slopes on the left should be MORE impactful than on the right, as teams played fewer games 40-50 years ago than they do today so they had less opportunity to gain or lose point differential. +50 points over 10 or 12 games is far more impressive than +50 points over 16 games.
However, there is also the notion that football prior to 1970 was not nearly as competitive as it is today. There were a handful of teams that dominated an era...and then you had a bunch of mediocre teams that they routinely beat like a drum. That would make a slope at the right side more impressive.
So, as usual, statistics can be successfully argued either way. Let's just be happy with the fact that Green Bay's line is currently on top.
Games per year do matter. And that would add to the effect of scoring enhancements. I think dominant team eras though stand out in the graph visually quite well.
Bud Adams told me the franchise he admired the most was the Kansas City Chiefs. Then he asked for more hookers and blow.