I agree that their numbers are on their way down, but I doubt the numbers will go down rapidly like they have in the past (Mass exodus after failed 1914, failed 1925, etc).
The past mass exoduses were caused by major date setting failures and major changes in doctrine. The WTS has learned from its mistakes and now will not make the mistake of overt date setting. Regarding major changes in doctrine, the WTS no longer uses a central leader like Russell, Rutherford, Franz, Knor, etc... instead all doctrine is now decided by committee. This causes doctrine to become much more stable while still changing slightly when faced with blatant realities (1914 Generation change in 1995 after it became painfully obvious that the past doctrine was not true).
Since committees now control everything, the end result will be a very gradual decline in their numbers as more and more people become exhausted from the burdensome load the WTS puts on people.