How long does it take to learn a lesson

Many years ago, I was a soldier. During my time, we had 2 main weapons; the 9mm Sterling sub-machine gun and the 7.62mm Self Loading Rifle (SLR).
Shortly after I left the Army, there was much talk about a shift away from these to smaller bore weapons.
The theory went that it wasn't really necessary to kill people, it was good enough to merely wound them. Someone had calculated that it takes 2, 3 or 4 (don't remember the number) extras to take care of the wounded, thus putting a serious strain on the enemy's resources.
Bollocks! shouted Kelvin. I have never heard such twaddle.
At the time, the big emphasis was on the potential Cold War opponents, USSR & China.
If the generals had read their history thoroughly, they would have noticed that this pair had a history of wasting their own soldiers' lives wholesale. Well, with the huge populations each have, they could afford to.
The "let's wound them and grind their efforts to a halt" theory thus goes right out of the window.
If a soldier of the USSR or China was wounded then he would be left where he was. In fact, if he was a Russian soldier and he managed to crawl back to his own lines, he would probably be shot by his own comrades for running away!
Regardless, the hierarchy pressed on with their "good idea" and eventually came up with the SA80, which fires a 5.66mm round. When they had added all the high tech gizmos, it weighed around the same as the old SLR so was no benefit to the poor bloody soldier who had to cart it about (about 10 lbs with a loaded magazine).
It did have a nice high-tech telescopic sight though. Fat lot of good that was as the weapon was not expected to be used much above 300m from the target. If you couldn't see the target without a telescope at that range, you were possibly in the wrong job.
When the SA80 was introduced, it was plagued with all manner of problems with bits falling off, breaking etc.
God knows how many years later, these "issues" were fixed.
Good! Now they had a rifle that worked. Except for this 300m range thing.
Throughout the time of the adoption and development of this wonder weapon, all of our real and potential foes have stuck to the good old Kalashnikov, good for 400M or more.
The net result is that our troops have to get a bit closer to the baddies, either by sneaking up on them or through the good old fashioned typically British tactic of charging the bastards.
Now some bright spark has determined that what we really need in places such as Afghanistan is a weapon more comparable to the Kalashnikov, so they have produced a new weapon.
Guess what? It is a 7.62mm self loading rifle.
Well, bugger me!
Isn't that where we came in?
Twenty odd years and who knows how many wasted lives later, we are back where we started.
Anyone who has served with the old SLR will tell you that when you get shot with a 7.62mm round, you stay shot. At any distance up to 600m or so.
Why do we need to re-learn all old lessons every few decades?
Why do we need to fix something that wasn't broken?