Garrett I like your car analogy. If you apply it to the key teachings of the religion when it began it goes something like this. The Bible Students started preaching that we are living in the time of the end that Jesus prophecied about and therefore we all need to leave Christendom and join them in the preaching work to survive Armageddon and save others.
So first take away that it's going to happen in one generation from the Kingdom being set up in 1914. Say that it is going to be within overlapping generations of 1914, which could go on for decades. Then say that what Jesus prophecied actually mostly applied to the first century. Then say we don't have to reach everyone because God reads hearts, so let's do lots of building work instead to make ourselves cosy in this system with comfortable branch offices, KH and assembly halls, which by the way can you all send money to us to pay for!
What you're left with is a totally different religion that has been changed piece by piece as you said. Now none of us are living in the time of the end at all. When they said millions now living will never die they were wrong and they're all dead. Either God got mixed up or he keeps getting his signals crossed to the FDS so they started too early.
Even staying within their own beliefs now those alive are not living in the time of the end and they've proved that they can't work out when the end is coming. They are all going to die and have to be resurrected to paradise. How is that different from any other religion? So why bother preaching? Why not live their lives and wait for the resurrection? What's the hurry?