The M16 has been in service since the 1962, the AK-47 has been in service since 1948. They're already old, but they are effective. Trust me when I say I don't think either will just disappear from the battlefield in the space of five years, weapons have longer life cycles than people give them credit for
Also, don't assume newer = better. Most newer weapons (Especially US/NATO ones) use 5.56mm in order to wound the opponent, while the AK fires 7.62mm. AK shots, needless to say, are generally more deadly if they hit. Neither is better, it's a trade off.
Now, the reasoning behind the 5.56 is that a wounded man takes more resources than a dead man. Good reasoning for fighting human enemies (Or not so much, if you have heard the reports that some insurgents are supposedly so fanatic that they continue to fight you to the death anyway) but not so good for fighting the Xcom aliens. Why? Well, do you think the Ethereals really give a shit that you wounded a Muton or Sectoid? Hell, would a muton even feel it?
The Ethereals seem like the type who'd be more willing just to let them die than heal them, and even if they aren't they probably have the tech to heal them up and pump them back out asap (They can genetically engineer species, have plasma weapons, cybernetic implants and anti-gravity/reverse gravity technology. I think they can probably heal up their troops pretty fast xD Though, we won't know this about the aliens at first, it'll likely be a bit of a painful learning experience when we realise the alien logistics aren't being effected the way one might hope when wounding their soldiers. Maybe a way of revealing this might be to find a sectoid who just got dumped and left for dead because he got wounded?)
Of course, 7.62 is considered less accurate and has more recoil than 5.56.
But your mileage will vary depending on the situation. Berserker charging you? Damage. Trading fire with sectoids who are in cover? Accuracy.
It'll really be the players decision to look at such a trade off
Also, don't assume newer = better. Most newer weapons (Especially US/NATO ones) use 5.56mm in order to wound the opponent, while the AK fires 7.62mm. AK shots, needless to say, are generally more deadly if they hit. Neither is better, it's a trade off.
Now, the reasoning behind the 5.56 is that a wounded man takes more resources than a dead man. Good reasoning for fighting human enemies (Or not so much, if you have heard the reports that some insurgents are supposedly so fanatic that they continue to fight you to the death anyway) but not so good for fighting the Xcom aliens. Why? Well, do you think the Ethereals really give a shit that you wounded a Muton or Sectoid? Hell, would a muton even feel it?
The Ethereals seem like the type who'd be more willing just to let them die than heal them, and even if they aren't they probably have the tech to heal them up and pump them back out asap (They can genetically engineer species, have plasma weapons, cybernetic implants and anti-gravity/reverse gravity technology. I think they can probably heal up their troops pretty fast xD Though, we won't know this about the aliens at first, it'll likely be a bit of a painful learning experience when we realise the alien logistics aren't being effected the way one might hope when wounding their soldiers. Maybe a way of revealing this might be to find a sectoid who just got dumped and left for dead because he got wounded?)
Of course, 7.62 is considered less accurate and has more recoil than 5.56.
But your mileage will vary depending on the situation. Berserker charging you? Damage. Trading fire with sectoids who are in cover? Accuracy.
It'll really be the players decision to look at such a trade off