It's a little shocking to see some of the mainline Christian religions with such drastic losses. Really interesting. I live in an area where many people still attend church, maybe not every week but still support those organizations. And there are many of the more conservative religions active (US). That's likely why my perception is that while they have dropped, it hasn't been terrible.
I think the more conservative religions foster a mindset that they are the correct path which I believe builds more loyal members. If you couple that conviction that you are the only path with the drastic shunning that JWs use, you can see why they aren't losing as many members.
I agree this religion (and most) is boomer driven and I think once they start passing away, you will start seeing swiftly declining numbers. Go to any convention/assembly. You'll see a large number of elderly and middle aged. Not nearly as many kids or even younger (20/30s) married couples who actually have or will have kids.
Credit where credit is due, the JWs are keeping their numbers up pretty well despite putting minimal effort in retention other than the shunning policy.