"since they really only destroyed two `elderly` Merkava`s in Lebanon" If only two were listed as destroyed, much more were disabled by anti-tank-weapons (and that's what counts). Also Abrams MBTs were destroyed in Iraq by IEDs. The unvulnerable tank is as much a myth as the unsinkable Titanic. Tandem hollow charge warheads have picked up much of the headway that reactive armour had gained. And as to cost-effectiveness, compare the price of a RPG-29 with that of a Merkava IV MBT ...
Two mortar shells explode in Golan Heights, no casualties or damage reported (Haaretz)
from the article: Revolutionary Guards: Iran can penetrate Israel's Merkava tanks