People evolve all the time. Average productivity growth (which measures how much people become better on average) is around 2 percent per year according to world's leading experts in the field.
How much of that is due to technology and how much is due to people changing their habits to do something more efficiently [or, even more radical, to do something that previously was impossible]?
The same or better growth applies to tennis level on average. Why? Because, it's human nature to learn and do better. It's human DNA.
It's also a human characteristic to be lazy and to resist change. The dynamic between the two determines how quickly change is accepted.
During times of feast, the motivation to change is probably very low. "Don't fix what ain't broke!"
During times of famine, the motivation to change is probably very high. "What we're doing is not working. We have to try something else!"
Though playing level may not change from 3.5 for most, the quality of play improves every year. 3.5s of today play vastly improved version of tennis than 10 years ago for example.
I have no data or experience to agree or disagree with that claim. I've been playing USTA for just under 10 years and I can't say I've noticed any NTRP being markedly better.
I do observe that, outside of tennis, there is "grade inflation": "C" used to mean "average" [which most people are]; now it generates wailing and gnashing of teeth.
That's real and that's progress. The level change from 3.5 to 4 is indicative of a player vastly outperforming others in his group. A level change is not the only indication of progress.
You can observe the same with pro tennis - they hit harder, move faster than players did 10 years ago. Did the ranking change? No, but they play much improved tennis.
Yes, there's a lot more emphasis on factors that weren't previously: conditioning, diet, sleep, mental health, "off season" training, etc.
There's also a lot more money involved: that has to change motivations.
However, if one could come up with an absolute measure of performance, it would be interesting to see how it changes over time: it may not be a smooth, linear ascent but could be jagged with even some declines along the way.