IMHO, a "mature" religion that is still alive doesn't plateau, especially since the "statistics" have played a significant part in the JW doctrine.
First, consider the "mature" Roman Catholic Church. Not a favorite in the UK, and definitely not unaffected by the general turn from religion occurring across the globe, it is still growing at about the same rate as the population growth, at around 17% in the current figures. In the UK the growth is at 2% whereas a much larger surge in growth is currently occurring in Europe and also in the USA. Around 300,000 new adult Catholics enter the Catholic Church each month on average, and the numbers on growth are measured against deaths and members who leave as the Church keeps track of these things (unlike the Watchtower).
People aren't "storming the doors" to become Catholic, but for a 2000+ year-old religion that began the 21st century by being rocked by a very public horrific child abuse scandal and currently facing an international decline and distrust in religion, it is doing quite well. That is what "maturity" looks like in a religion.
Nobody really cares that much about the child abuse scandal in the JW religion (which is unfortunate for the victims). It is a tiny relgious movement, an annoying cult, and people in general expect cults to have sexual predators and perverse dealings (look how long the sexual abuse in Reform LDS stayed in the news and remains in people's minds now...not at all). The JWs have never been taken seriously on a world scale. Nobody considers them the seat of Christian Faith like they do Catholicism, so nobody cares what is happening to them. No one outside the JWs has really bothered to notice they are still here past their "due date," so to speak. And after the Watchtower sign comes down in Brooklyn Heights, people are going to say: "Oh yeah. Whatever happened to those weirdos?"
So it's not the world causing them to slow down. It's not the sign of maturing and becoming a common fixture on the relgious scene. No, this is what failure looks like.
We used to have JW assemblies in the same meeting complex at the same time the Worldwide Church of God had them. One day they stopped having their assemblies in the same place. Why? Because of what we are seeing with the JWs. This is the same thing that happened to them after Armstrong died and the world didn't end. The money, the consolidation, the lack of growth. It happened just like this. The next step is for them to do what the leadership of the WCG did, announce to their members that "yes, we have been in a cult." And boy, that ended it all for them, right quick.
The end did not come. They won't be able to keep it going much longer no matter what, because the end didn't come and it is all based on this being the end. The JWs can't exist if this isn't the end ("true religion" will only be restored for the generation before the end of the world, according to the JW theology) and the end never came when "Jehovah told them" it would. The "growth" numbers didn't mean what "Jehovah told them" it meant. None of it came true on time, and that time was over a long time ago.
The sad thing is denial can last a long time. When it is all finally over, the world will notice as much as they did when the WCG went belly-up, which is not at all. What a waste this will all have been. No one will even care about the years or time I wasted either, how sorry a lot is that?