You can kill elephant with infantry/AT builds. If you see one on the field you are very likely to spend more resources into AT/infantry department and just kill it. ISU, just like many Soviet units, just follows the pattern of being the jack of all trades. Soviets don't really care about analyzing what is on the battlefield. Whatever they build will be ok against most threats. There are people building ZiS guns just to act like mortars even when there are no vehicles on the field. They don't risk wasting resources by building wrong counters. ISU follows the same pattern. Axis armies don't have such luxury. If you build an elephant, the enemy will win the infantry war.
You just decribe gamedesign differences between OST and SU. OST have units with narrow specialization (better perfomance against specific targets), while SU units all-around (mediocre perfomance against 2 types of threates). OST have only few all-around major units: P4 and Tiger. And SU have few specialist units: katy and su-85.
You can kill ISU use infantry/AT builds (shreck blobs, stun shots, blietzkfrieg armor) or use your elefant. In 1vs1 there aren't unbeatable builds.
I saw that game with zis spam. OST player lost 2 brummbar until he realized that against team weapon spam he need werfer...It was tunnel vision of OST player.