The last tank the position of "alternate driver", "co-driver", hull machine-gunner, was used was on the m47 Patton which was derived from the m46 Patton which was derived from the m26 Pershing. The soviets similarly do away with the position after the T55. The main purpose of the "alternate driver" was to operate the hull mounted machinegun. In WW2 the combatant armies decided the hull mounted machinegun wasn't as effective as previously thought. The idea had been that the machinegun would be effective against infantry troops. However, in practice the machinegun had limited ability to cover the entire area. It was nearly as effective to use a gun mounted parallel to the main gun. Additionally the hull machinegun created a weak point in the armor of the tank and required an additional crewman.

The M48 Patton was the first US tank to do away with the hull machinegun, It was also the first tank to receive the title "main battle tank." This was largely because as stated above the M46 and M47 were mostly just modifications of earlier designs. The M48 represented the first complete design overhaul after WW2.

One odd bit about the M48 Patton removing the hull machinegun was that Patton himself was a fan of the gun. Patton liked to use the gun to fire into areas were he thought an infantry ambush might be waiting. Kind of like scouting using a gun.

So both of your theories are correct in part. At the end of the day the gun just wasn't effective enough to warrant the additional cost.

The issue with tank crew sizes works out pretty simply: More crews = more armor. More armor means bigger, slower, heavier tanks, which cost more to make and more to keep moving. During most of World War Two, tank armor could match many of the rounds shot at it. Further, tanks like the KV, IS, Tiger, and King Tiger embraced the concept of a "heavy" tank.

Following World War Two, however, tanks were increasingly designed to strike a balance between firepower, protection, and battlefield mobility (both tactical agility and operation flexibility). Further, the US and USSR pioneered APDS rounds and HEAT shells, which were cutting edge technology during the war. These new designs were very good at penetrating solid steel armor.

So designers faced a dilemma. The amount of material required to protect a tank crew was quickly reaching unacceptable levels of weight. With traditional designs, the large crew space would have forced designers to utilize a large amount of thick metal to protect the huge space. This would make the tank larger, heavier, slower, and more expensive. Or, designers concluded that they could merge positions and eliminate superfluous positions, (not just assistant drivers, but radioman, 2nd loaders, and other positions. Some designs even tried to replace loaders and gunners with television controlled automated turrets). Smaller crews mean smaller tanks, while maintaining overall protection.

I think too there was a general realization that the larger crews didnt appreciably increase efficiency. Radios became easier to use, and small enough that a commander or driver could work the radio. Rounds generally were standardized at 75-120mm, small enough for one loader to use. And the machine gun station that the Assistant driver could similarly be either eliminated or automated. All this improved tank characteristics.