Yes, it is quite possible Iran would massively retaliate. But there comes a point in the minds of Israeli politicians that the threat against Israel will only increase as more time goes by. They may have asked themselves: Will Iran be stronger a year from now? Ten years from now? Is it better to attack now while they are not as strong, rather than wait and face even a stronger, more unpredictable Iran? These are the hard questions that Israeli leaders had to ask themselves in secret meetings.
I don't know the answer, but judging by past actions, Israel attacks preemptively, decisively, and makes sure the targets are taken out so they're no longer a threat to Israel. This happened against Iraq and many other neighboring Arab nations.
Since the holocaust of WW2 the Israelis are not prone to wait and allow an enemy nation to build up a war machine that could be used against them. The safety of Israel only exists if Israel PREVENTS an attack before it can take place. I don't believe the Israeli leaders will quietly stand on hope, that an adversary that is building missiles and nuclear weapons, won't someday at their choosing incinerate the cities of Israel.