Glad you got a great car seat, and specially one that will rear face your child until they 40 pds.
Ah the eye rolling, is my personal favourite! If you posted on the car seat site here, you probably have your answer and good links above.
Think of it this way: if you were in a car crash, you take your body weight x mph = force of crash. So if your baby weighs 23 pds and you were going 55mph, the average speed limit where I live. That is now 1265 pds of force that your baby is going to be hitting their harness that they are strapped into.
Now, you have to remember that a babies heads is usually on the larger size, so there is more weight there. Think of their head moving (this is all in a front facing position) forward at 1265 pds of force and then slinging back into the seat after the harness has restrained them. Internal decapitation happens when the spinal cord is stretched more than 1/4inch.
When a child is rear facing, the force of the crash is distributed over the back, neck and head of the child (as opposed to the just the harness in a front facing child). Therefore the risk of injury to the spinal cord is less because the impact is spread out more. Whereas the area of impact is just where the harness is holding the child when front facing.
I am not sure why people are so eager to turn their children ff'ing at 1 year and 20 pds. It's almost like a milestone in a babies life and it shouldn't be. For every step that a child takes in car seats, is a step down in safety for them.
Here is a good site for information for those husbands/parents etc who want to roll their eyes at rear facing:
Car Seat Safety: Rear-facing is safest
There are good links to more info, test sites etc for you to check as well.
Kudos to you for wanting to keep your baby rear facing for as long as possible!

Car accidents are the number one killer of kids today, and we should be doing everything we can to prevent it.