You know this, how? Not being privy to any attack plans it is a stretch to say immediate multi front war would result. You are forgetting the major money players who want Iran out just as they wanted Iraq out...the House of Saud. An effective attack would cull the lesser players who are well compromised now or they would have done something long ago. It is possible to both destroy the program sites and the leadership. Russia wouldn’t be happy, though there are factions there who would welcome the political change w/out Iran in the Politburo.
To answer your question, How do I know this:
Common sense tells me Hamas & Hezbollah need little provocation to attack Israel. They both get arms from Iran, so they owe Iran some allegiance. Syria is a partner of Iran, so I fully expect Syria to come to Iran's defense. Syria also wants the Golan Heights back. They can't get it back by themselves. Syria also presents an opportunity for Iran to wage a ground war on Israel thru Syria.
If any of these 3 turned their back on Iran after an attack by Israel, it would be a great victory for Israel & bring shame on the 3 cowards. Muslims don't react this way, & they would rather die than be shamed.
As for the Saudis, they never get involved militarily until they are invaded. They will play both sides until a winner is certain. Don't look for them to do much more than ignore Israeli overflights & spread a little money around to gain favor with everybody involved.
Short of using nukes, I do not believe that Israel alone has the military assets to destroy Iranian nuclear ambitions, only delay & disrupt them. And this strategy ignores chemical & biological weapons that could be built,deployed, & used much easier than nukes.
If the US were to join in, it could certainly be done. But that's not happening either. GWB didn't do it, even with all the provocations Iran made during the Iraq war. I doubt wimpy Obama would do it even if Iran sank one of our ships.
OTOH, the mullahs, Imanutjob, & a few others are the SOURCE of the danger facing Israel from Iran; & until they are dead & replaced with more rational & reasonable people, the danger to Israel will always be there - nukes or no nukes.
Much of this could be done w/o all the risks of a direct attack.
While it “is possible to both destroy the program sites and the leadership”, it is also possible to fail, & lose the cream of the air force in the effort, which could leave Israel weak to repel an attack from nearer home.
Nothing I would like better than to see ANYONE destroy the mullacracy & their nukes, but the risks are very high & success is not certain, nor is the reaction of the rest of the world. Only the US could do this & “get away” with it in the court of world opinion. Israel doesn't have a chance in that corrupt, immoral arena.