They cannot knock out every single base in one wave. Even Iraq in the Gulf War faced off in dozens of air to air sorties against coalition aircraft, and the country is much smaller.
If all the Migs engage is cruise missiles and drones, then that in itself is valuable deterrence. Nobody expects Mig 29s to fly into Iraq and dogfight with the Israelis. They need to provide point defence against drones, protect our TELs and missile bases from drones and missiles.
At the very very least, force Israeli aircraft to perform evasive maneuvers, taking them off target, giving transport and launch crews valuable time, allowing air defence to track and engage targets.
If the choice is nothing, or Modernized Mig 29s, I would still choose the latter.
Lastly, If the regime survives, we would need CAS for suppressing insurgent groups, Taliban, maybe even rival arab states. You never know. When all the dust settles, if we have Migs flying over Tehran, it's a big boost to morale and a sign Iran has endured, and has not fallen.