i mean these tanks will be gone within 20 minutes if any one attacks they belong to 70s and 60s.
Iran's defense doctrine is asymmetric warfare not classic warfare i.e Kursk style classic tank battle. Specially when a small county is facing a much stronger adversary who's favorite tactic is to destroy the air defense as well as other vital defense assets with "unstoppable" cruise missiles and then bomb the rest of the country for awhile before it's ground forces even get into action.
Regardless of this fact, Iran or any country for that matter, still needs armor in it's army to discourage local threats (such as Taliban incursions against Iranian border towns and the murder of 10 Iranian diplomats and one Iranian reporter after the capture of the Mazari Sharif city in northern Afghanistan in 1998 which resulted in deployment of 70,000 Iranian troops to stop the Taliban advances).
Cheers.