>How did Western Christianity come to focus so much on going to Heaven when your body dies, instead of focusing on the Second Coming, Resurrection, and New Earth?
And? Yes, there needs to be more focus on the second resurrection and new creation, but why does that mean removing the intermediate state?
>But when I read the scriptures, there isn't so much of this in there.
Like the Sadducees of a passage I'm studying intently, you neither know the power of God nor Scripture.
>Paul talks of those asleep
From outward appearance. This says nothing about the current state of the physically deceased's consciousness.
>. Christians today act as though once you find salvation and peace with God, and your body dies, you've won the race and that's it.
That's an issue that involves more than just eschatology, so using yours, even if correct, will not fix that attitude.
>This also ties in with the rapture theory,
That's a different topic entirely, irrelevant to the intermediate state of a person or the nature of the third Heaven. But yes, dispensationalism has been destructive to evangelical Christianity, especially in America.
>the righteous bodies will be snatched up into the spiritual realm forever (where their souls already are) to live forever while the physical universe is utterly abandoned & destroyed.
That's not what futurists believe. The rapture is a pre-Tribulation (i'm a post-millennial, orthodox preterist btw) event where the dead saints are risen from the dead and Christian who are alive at the given time are raptured with those now reembodied saints into heaven or wherever Christ wants to meet this group. It's basically the resurrection before the resurrection. Also, if you are physically alive, your soul and body composite, your soul will not be somewhere where your body isn't unless you die.
>What's the benefit of having a body in a purely spiritual realm?
That's a good question to ask the now bodily-risen Christ who sits on the right hand of the Father.