It mostly comes down to aesthetics.
If you really like your $1500, you won't notice much of a difference on a $6K bike, unless you are really OCD.
When I'm out riding, I often literally forget which bike I'm riding- my 18 year-old aluminum bike, or my <2 year-old carbon-fiber $4300MSRP bike with $800 worth of upgrades. (I bought it used for substantially less!)
In the end, it's more about the rider than the bike.
With bikes, it's much like: A Timex will keep just as good time as a Rolex; yet some people are willing to spend tens of thousands of dollars for a Rolex. It doesn't do anything more than the Timex....it just looks and feels nicer; and is something to admire.
By $300 Bikesdirect first adult road bike shifted and performed perfectly adequately. I like my fancy-pants modern bike O-K, but I'm glad i didn't buy it new for full price....because in the end, my rides are the same.