This is a drawing of the A1E1 “Independent” heavy tank design. It was a spectacular prototype in the 1920’s, back when “heavy” tanks were called so becuase of their relatively weight without expecting thick armour or big guns.
It was heavy not becuase of thick armour (it was merely bulletproofed), but because of its big crew. Four dedicated machinegunners were needed to operate the four small machinegun turrets.The tank was means to be able to fight infantry all-round on its own. This design philosophy was not very successful commercially during the Interwar Years in part becuase it made the machines very expensive, and it was incompatible with the later adoption of shellproofing armour. The area to protect was simply too large with such a large vehicle, any shellproofing (60 mm and more steel plate thickness) armour would make immobilise the tank by breakdowns and sinking into soft soil.
more importantly, the whole idea of what a tank should be was wrong. A tank does NOT need to be able to fight in many directions simultaneously, for a tank is not alone on the battlefield. Infantry to the left or right or in the back can and should be dealt with by allies, such as infantry and other tanks. A tank needs a main weapon (which can even be casemated with limited traverse, but is usually in a 360° traverse turret) and one low effort all-round machinegun, which would actually rarely be used to shoot sidewards. A machinegun coaxial tot he main gun has become customary as well, as it offers much for little effort and can easily be reloaded under armour.
That’s the typical main battle tank today; 105…125 mm main gun in 360° traverse turret with coaxial machinegun, one all-round machinegun on top of the turret. The hardware follows reason, but the idea of a tank as a force of one, capable to fight on its own, that one is too enticing. People still fall for it, especially laymen.
The Russian tanks (which are near-identical to the Ukrainian ones) get destroyed in quantity, even with anti-tank weapons that should not be considered anti-MBT assets. This is possible because a shot from the flank can penetrate without a very powerful warhead and cause a catastrophic detonation of the main gun’s munition.
The side armour is rather meant to protect against powerful weapons from the frontal 45…60°, not from the flank itself. The angle of impact adds to the armour’s effectiveness.
The forces in the field were and are supposed to mitigate this problem of flank vulnerability, not the tank designers. The tank crew of a lead tan should be able to rely on infantry and/or fellow tank crews to secure its left and right flank, so it itself can focus on the front and orient its best protection forward.
A hard kill active protection system may reduce the need for such cooperative tactics, but only so as long as it works as advertised. Hard kill APS will likely tempt even the professionals to think of a tank as all-round fighting machine, conceptually taking the place of the four machinegun turrets of the “Independent” tank prototype.