For me, the difficulty in this argument, in all sports, is always related to how would each generation adapt to the conditions, equipment, and competition of a different era?
Top players of any generation tend to be as good as they need to be to reach their goals - grand slams, #1 etc. When that is achieved, there is no real incentive to become 100% better, to move the game forward exponentially, as they are already able to achieve their aims by outperforming their peers. There is no added benefit to be gained from adding to that burden, to say, at one logical extreme of 'excellence', aiming to win every match 6-0 6-0, to never lose a point.

They are rare individuals indeed that reach the very very top, and still want to go 100%, 200% further. None of them like to lose, and they want to win every match, but, generally realise that it's impossible. As the peerless Serena Williams - at this stage in her career, amongst the most clinical and merciless winners in any sport in my memory (special mention for Jahangir Khan & Esther Vergeer) - said at the Australian Open just past, "Every time I walk in this room I'm expected to win. I'm not a robot. I do the best that I can. I try to win every point but realistically I can't. Maybe someone else can.".
So players do what is necessary in their era to win and reach the top of the game, they don't do what is necessary to compare to some future standard that is unknowable to them.

Standards of fitness, tactics, the range of shots and how and when they can be played with the equipment of the age (there's a reason Borg didn't have Rafa's extreme forehand, and it wasn't because he couldn't have executed it had he had the frames and strings that we now have), amongst other things - the general manner of play and nature of the game - are all constantly evolving, and, broadly, improving.

The question that is so unanswerable is: If those players from generations past were raised in this generation, and had grown up with all the same things, culture, methods as the current players, would their natural talent and drive cause them to naturally raise themselves to the - undoubtedly higher - standards of today?
There's no right or wrong answer to that question, as it's really a question of the unknowable quality of human temperament and motivations; both very poorly understood aspects of the human experience.

As a final counterpoint, I should just mention that this works in reverse too. It is equally unknowable whether any modern player would have taken so readily to tennis if they had only ever been exposed to less forgiving equipment and the conditions of previous generations. Perhaps they would not enjoy some thing as simple as not being able to thump every single ball with massive swings, and so on.

All I ever really say is that each player in any sport that achieves great success was uniquely suited to the conditions that arose whilst they were playing, or for at least a part of that time; even most of the greatest players did not have continual and uninterrupted triumph through the entirety of their careers, they had 'fallow. periods - not for want of trying.
For me, personally, that's more than enough.