7.62x39 has it's uses as the US military itself has found out, which led to all sorts of experiments like 6.5 Sneedmore and .300 Blackout and a whole host of other calibers. It's more usable suppressed, it's better at barrier penetration, and so on and so forth.
Plus Russia has a shit ton of 7.62x39 ammo in stockpiles, why wouldn't they keep it around?
What compatibility issues?
Yeah we haven't gotten into the other calibers yet, we're still on weapons in active service in the US military that use 5.56. Are you ready to start adding stuff chambered in 7.62 NATO to the list?