Old tanks would fight old tanks or soft targets then.
They wouldn't survive long enough to get to the battle field. Russia kept all of the old tanks for use as barter for their proxy country's they used to be allied with, additionally, they had a tactic of trading armies for a short while, the theory being they wipe out your army while losing theirs, they then trot out their second army equipped with old stuff, but because you have nothing, they win.
The problem with that tactic is they ran in to the real world and our tanks are MUCH better than those they field, and their first, second, and even third armies would be toast. So now, they are trying to build one good army that can survive a battle with ours.
M1 is not that great and it is decades old, too. A T-72 can destroy it within 1000 meters (Iraqis had problems because they used their own ammo). While the M1 has a good targeting system, its armor proved to be vulnerabel to old soviet rpgs and explosives. While the M1 can be recovered when not destroyed, the process is slow and no new tanks are produced in the US.
That is a laughable assertion. There are MULTIPLE accounts of T-72's hitting Abrams tanks at ranges of 100m and less, and having the 120mm RAPIRA rounds lodge in the armor. One Abrams crew thought they had hit a mine until they got hit a second time. They slewed the turret, found the offender and blew him to bits with a single round. The Iraqi Army Abrams tanks are not equipped with the DU armor Suite that American tanks are so they are more vulnerable than our tanks, that's the only reason I can fathom for you making such a ridiculous claim.
You have learned little to nothing about non-american arms. The Soviets were the first to mount smooth bore cannons. The 2A46 cannon can fire a multitude of ammunition types including both shells and missiles and is stabilized to some extend.
"The T-72 employs the same armament, ammunition, and integrated fire control as the T-64. The low, rounded turret mounts a 125mm smooth bore gun with a carousel automatic loader mounted on the floor and rear wall of the turret. The 125mm gun common to all the T-72 models is capable of penetrating the M1 Abrams armour at a range of up to 1,000 meters. The more recent BK-27 HEAT round offers a triple-shaped charge warhead and increased penetration against conventional armors and ERA. The BK-29 round, with a hard penetrator in the nose is designed for use against reactive armor, and as an MP round has fragmentation effects. If the BK-29 HEAT-MP is used, it may substitute for Frag-HE (as with NATO countries) or complement Frag-HE. With three round natures (APFSDS-T, HEAT-MP, ATGMs) in the autoloader vs four, more antitank rounds would available for the higher rate of fire.
The infra-red searchlight on the T-72 is mounted on the right side of the main armament, versus on the left on the earlier T-64. The 1K13-49 sight is both night sight and ATGM launch sight. However, it cannot be used for both functions simultaneously. A variety of thermal sights is available. They include the Russian Agava-2, French SAGEM-produced ALIS and Namut sight from Peleng. Thermal gunner night sights are available which permit night launch of ATGMs."
T72 Tank Characteristics