Tennis was at a totally different level of development back then, the gap between top players and lower ranked one was way bigger than it's now. You have to take Chris's and Martina's stats with a grain of salt, their opposition was way weaker. If a player back then had a single weapon that stood out in the field, they were able to dominate almost everybody they encountered. That's also the reason why they played way more matches and won way more titles.
That's not true at all. You're just forgetting about all of the good players that were around. Martina and Evert played against a lot of people you probably haven't heard of who were excellent players. Plus, Navratilova especially had to deal with extreme depth: Graf, Seles, Capriati, Davenport, Pierce, Evert, Mandlikova, Sabatini, and so on.
Tennis seems to have more depth now because you're more familiar with today's game and players. When Martina first started she had to contend with Goolagong, who beat her in a lot of close three set matches, Court, and many others. She also played a ton of mid-grade players that no one remembers but who are like the Schiavones, Bartolis, and Stosurs of yesteryear.
Evert and Navratilova were so far above the rest because they were geniuses on the court. Evert was an incredible machine and Martina became one. But, even Evert -- in the middle of her record clay run, was beaten by Austin. She was also beaten after another record run by Mandlikova.
Players had very complete games. In fact, they had more variety back then, not less. Evert's mastery of the lob and drop shot were crucial to her game, and she could volley just fine -- especially at her prime with wood on clay. Navratilova was also very skilled with the drop shot.
Speaking of depth... even though Martina had an 86-1 season... guess who beat her. Kathy Horvath. Yes, the