No, thats not true at all (at least from what I've seen).
I know alot of people who USED to watch wrestling and don't anymore and also a couple who started watching it recently (most are casual, but a few are/were hardcore fans) and the #1 reason I'm given isn't because there aren't new fans (or former fans for that matter) to be had its just that they don't like what wrestling has become over the last while. Now the funny thing about these people is when something garners their interest (most recent example was when Bret Hart came back), they will turn on wrestling to see whats going on, which tells me that the fans are in fact out there, they just don't have enough reason to watch wrestling.
Now I'm not going to sit here and badmouth TNA and WWE because I understand the need for change (especially in WWE's case) but the fact of the matter is wrestling isn't nearly as intriguing as it was about 12-13 years ago. Most of these fans who watched in the late 90's early 2000's aren't watching anymore, but its not because they don't like wrestling anymore, its just because wrestling doesn't offer them anything.
A hardcore fan like myself judges wrestling on many different levels (in ring work, promo work, storylines, building of characters, main event, mid card, lower card, backstage, ect.), but your average fan judges it on 1 thing and 1 thing only "am I being entertained?". People found it entertaining to watch the nWo, people found it entertaining to watch Austin/McMahon, but your average fan is not entertained by the recent Nexus angle. Why? because they've seen it all before, and there is nothing about Nexus that is especially intriguing to the average fan. Even though the Nexus angle is being done very well IMO, your average fan sees it as nWo version 2, they don't care about building the roster or the future of the WWE, they just wan't to be entertained plain and simple.
If TNA finds a diamond in the rough and finds something the fans want to see, they will come, same goes with WWE, if they find that next Austin or Rock, there will be more fans to watch the product and the ratings will increase. Over the last 7-8 years ratings have been pretty stagnant with no HUGE jumps or drops in ratings (I'm talking about 1.0 jump in a week or something fairly significant), which shows the average fan isn't watching wrestling because they are more entertained by a show like the Big Bang Theory, but if either organization gives these people a storyline or a character they want to see, they will come back and they will stay until they get bored.