So in Summary: Athletes not improved (at least not in any significant way), their rackets improved but that doesn't translate to the player being improved.
I disagree on that point. The more multifaceted you get with regards to skills required to be competitive the more athletics events
have improved over time. The 100m sprint is right down the
one-trick-pony end of the skills spectrum with regards to skills - which is why it's regarded as one of the pinnacle events to succeed at (there is almost no margin for error or way to make up any shortcomings or screw-ups), but also why the scope for improvement is so small. The decathlon as a better example related to tennis and an event which has had significant improvements over the years (if the same scoring system is kept consistent). The guys now would wipe the floor with guys from the 80s.
Tennis is an incredibly multi-faceted sport requiring a variety of skills, learned movements, strength and endurance. The avenues for potential improvement are significant - but some have been focussed on more than others. For example, movement and speed amongst the top 10 has, on average, improved
significantly in the last 20 years. And, it's done so in players with the weapons to win regularly. Often, as the 90s shows, most of the truly great movers were otherwise under-gunned. Sampras and Agassi almost exclusively managed to nudge into the category such that it made their other strengths even more potent.
Squash is another sport in a similar boat. The very top players from the early 90s wouldn't even be top 50 today, perhaps not even top 100 - and racquet technology has hardly changed at all in that time for them (strings don't matter in squash like they do in tennis since spin is barely relevant most of the time). But it's not shotmaking that has changed - it's court positioning and movement. Consistence and speed compared to the 80s improved such that the scoring system even had to be changed (for men only initially) to shorten matches.
No-one would doubt that given the same upbringing in a modern era Borg and Lendl would have realised their tennis potential differently - both were mega talented players - but as they did play, even with a racquet change (and time to adapt), I just don't see them being competitive today.