This is one problem, certainly. But even if there were more footage, I believe the perception would remain among many people of an inherent “superiority” of the moderns.
The underlying problem is a reductionist perception of sports where the skills/artistry factor is too tied in with other factors that evolve through time such as equipment and physical conditioning, especially equipment. The tendency is so strong that it affects people even in their assessment of works that are considered strictly “art”, such as music, painting etc. (even the most hideous architectural monstrosities of the 20th century are perceived as architectural progress with respect to the past). There seems to exist this tendency to believe that progress is a universal force imbuing every form of human activity and mental process. It may be some kind of journalistic illusion that is force-fed to us from all angles.
It’s mostly BS of course, but it seems unstoppable. There are even tons of people convinced that we now understand how the world came to be, how life originated, how existence is possible...just because we are so much more intelligent than our dumb ancestors. It’s a kind of generalized self-delusion.
The range of human skills to direct a tennis ball with a racquet cannot have changed form an evolution point of view in 100 or in 1000 years. Those skills are the same, and they are just adapted to the available equipment.
The old equipment allows a more immediate appreciation of skill by itself, if you are able to keep in mind certain limitations the equipment offered. From this, some enthusiasts of the old game derive the mislead conclusion that skill itself has actually decreased. But that's not true either.
On the other hand, the intoxication with the notion of progress on all fronts among some of the modern followers causes them to believe that not only equipment and (to a degree) physical conditioning have improved, but that skill itself never ceases to increase.
One poster writes today:
Athletics never go backward. Today's players are always bigger, faster, stronger,
and better than any era in the history of the game of tennis.
http://tt.tennis-warehouse.com/showpost.php?p=7291916&postcount=7
And another one:
Anyone who watches tennis knows the players have been getting faster, stronger, g
enerally more skilled continually for as many years as we've been watching the game.
http://tt.tennis-warehouse.com/showpost.php?p=7293628&postcount=24
There isn't much you can do about this. These beliefs in the progress of everything just won’t go away. They are in the air as a kind of continuous bombardment, and some people absorb them very easily. And for those who do, watching old videos of great players would just confirm what they already believe.