I haven't read any official figures about how many people play tennis these days and how many watch tournaments like the Australian Open on television, but my perception is that tennis became very popular in the '70s and into the '80s and then fell back into its traditional position as a minor sport in popularity.
Whether Connors and McEnroe caused that popularity surge or were the beneficiaries of it is hard to say. Probably some of both. It seemed to start with the emergence of Chris Evert -- who had a shy, undemonstrative personality to put it mildly -- but who was also attractive and feminine, a most pleasant contrast to arch-feminist/lesbian Billie Jean King. Evonne Goolagong, who was part white and part Australian aborigine, was also quite popular during her short heyday.
Bjorn Borg was like Evert in that he was very introverted, but he was almost as popular as Connors and McEnroe despite not being an American, especially among female tennis fans.
Despite all the media hype given the Williams sisters, tennis has sort of stayed at the same level of popularity for the past 20 years. Andre Agassi is very extroverted but there was no "going to the next level" during his long career either.
It may have been a "baby boomer" thing. Tennis is a great sport, but you have to be fit to play it. I was really into it myself during college and was a pretty good player. But once you hit middle age you have to be very dedicated to still be able to play decently. It's not a sport like golf that you can play your entire life. My guess is that as a lot of the people who began playing during tennis' popularity surge in the '70s and '80s stopped playing, the interest of many in following the pros waned as well.