I object to the OP on so many levels, i think I may have misunderstood it! or rather, the OP and I are so far apart in our outlook on life that its hard to know how to respond!
Many good points already made, especially about what happens to "gifted" 5 year olds later on...so many things can affect a child's development and decisions, and so many children change so much as they grow, to worry about society because "gifted children" don't fulfil their potential seems pointless. Some people are doing well and filling the top jobs, so it hardly matters whether they were deemed gifted as youngsters. Perhaps the wrong people are in those top jobs, but it doesn't mean that the "gifted" children would be the right people!
What you end up doing in life is so much more to do with what influences you, what experiences you have, your motivation and your drive, your personality, than whether you learned to read at an early age or were a whizz at maths. And one person's fulfilment is another person's pugatory.
Your final para, OP, I found the most bizarre:
"my little average achiever gives me the most joy and the least worry! I know he will be happy and fulfilled come what may."
..not only is it patronising but also very naive....why will he be happy and fulfilled come what may...because he's not bright enough to know the difference? This sounds brutal, but you don't have to be gifted and talented to feel frustrated and disapointed about life!
Really don't want to attack you OP, but bewildered by where you're coming from!