Put yourself in Afganistan. Pick up your rifle. Its an SA80 rifle. Its not got bad accuracy up to maybe 300M or so, but for every metre beyond its working range its getting more powder puff.
In addition, any Taliban hiding behind anything solid (and a lot of the Afgan walls and buildings have quite solid walls) can't be easily dealt with by 5.56. However, you may die, because the 7.62 round he is firing at you is going through objects, walls and cover. Including the one you might be sheltering behind.
Further, the silly 5.56 is designed to tumble when hitting (I'm over simplifying this) and its supposed to injure and main and cause a lot of medical support issues for russians. Its not fundamentally a killer by design that is needed in some places like Afganistan.
The 7.62 on the otheer hand is going to make a very big mess of you or kill you.
Some of the justification for 5.56mm in logic is fine. You can carry more of it, and its lighter per cartridge, and its building, warehousing and production may cost less. All of this is fine if you are a pointless accountant sitting in the ministry of silly walks. On the field a soldier wants a round thats outgunning the enemy, and killing them. And if its very good at doing so, you can carry less ammo.
You don't have to give every man in a unit a heavy weapon, you just need to rebalance the units so they have the right mix of light, medium, heavy and long range and support to win any and every fight.