And I'd like to state that armoring a MBT's side verses 'heavy' threats isn't unfeasible. You wouldn't want to use Heavy ERA for the sides because it's protection is diminished without the angle that's imparted to it if it were placed on the front vector. Heavy ERA on the side of a tank is rarely sloped, so it's protection is diminished. Despite this however, Western tanks in particular have proven that up-armoring their sides isn't unfeasible. But armor isn't the only necessity, crew survivability also is. From what we know of tanks to date, the most survivable tank of all time, the M1 Abrams, only became that way because of it's totally segregated section of ammunition. Other large western tanks, like the Merkava or the Leopard 2 or the Challenger 2, also have a segregated ammunition section, yet they still retain ammunition in non-segregated parts. The Abrams therefore, remains to be the only tank in service to have a totally segregated armor section, which is crucial to preventing a complete destruction of the tank in case of a penetration.
And even so, penetration does not necessarily mean a kill. For example, a PG-29V warhead, with a penetration of anywhere between 750 mm to 1,000 mm of RHAe, was able to penetrate the side turret of an Abrams but was unable to detonate the ammunition stowed in the turret. This was due to the fact of the Abram's relatively thick side-turret, which is estimated to be more protective than the glacis of the T-72BM, without Kontakt 5, which is, considerable to say the least. Similarly, one can field Active Protection Systems to either stop or slow down a projectile, and have the main armor to receive the brunt of the force.