Immortals are still cheaper than MI. As the persians, if you hit an early, despotic golden age and use it to expand, and build the Great Lib, than you can use the Gold earned from not researching to hurry improvements, such as marketplaces, and etc. And once education is learned, cheap universities. With immortals you can cripple anyone near you, specially foes that have strong UU later in the game. Weaken and cripple all your neighbors so later you can finish them off. You can accomplish that all in the ancient age. Later with hurrying marketplaces and other improvements and building cheap Lib, and a large empire no one can catch up to you in tech race (even if they somehow trigger their golden age). The point being in warmongering, an early advantage in military and amount of land (size empire) assures an early domination victory, even before you reach military tradition. And immortals as an offensive unit become obselete only when Riflemen show up.