I think we should not draw conclusions too early. The firing ports, even though the only allowed a limited field-of-view (and usage), still did have some advantages.The main advantage of firing ports was and still is that they allow the crew to fire their weapons, even if the area has been contamined by nuclear fallout. Other vehicles which doesn't have a turret with main gun and doesn't have firing ports, like the M113 are not capable to defend themselves, unless the whole crew is wearing special equipment (which does protect for a shorter frame of time than a NBC protection system of a vehicle). But we should not forget that without firing ports in most cases the crew can't fight from inside the vehicle (which should actually question the classification of the vehicle as IFV) or has to use hatches to fight enemies during mounted combat. Hatches does reveal a far more from the infantry than firing ports (firing ports reveal nothing), which means that a close MG gunner/normal soldier with assault rifle can kill members of the infantry squad.
The US, Germans, UK etc. all did decide to remove them on later generations as the situation changed. When the BMP-1 entered service, no medium vehicle was capable to resist a hit from it's HEAT rounds (at the beginning the only ammo carried). But the gun had a short effective range (< 700 m), which lead to the requirements common in NATO that IFVs should be capable to penetrate the BMP-1s armour at ranges greater than 700 m (20 mm AP can already penetrate the majority of the BMP-1s frontal armour at 1,000 m). Therefore M2 Bradley, Warrior, AMX-10 (probably) were all only capable to resist 14.5 mm rounds. The Germans somehow wanted a higher level of protection for the Marder, which made it the heaviest IFV back then. When the BMP-2 came into service, it was capable of penetrating every IFV in the NATO (14.5 mm resistant at 2,000 m with AP, Marder probably with APDS at 1,000 m). This meant that NATO forces where in a bad position, but there was a solution. While it was impossible to overcome the penetrating power of the 73 mm HEAT round with then existing armour, 30 mm AP(DS) could be stopped by up-armoured IFVs.
The side armour, even though 30 mm APDS can still penetrate it with a 90° impact from most ranges, was reinforced to increase the level of protection for +/- 20-30°.
Btw: ICV might not be the best acrynom. It can stand for Infantry Combat Vehicle (= IFV) or for Infantry Carrier Vehicle (=APC).