One way or another, they would probably find they can manage fine. Iran is no scarier than Mao's China was in 1964, when it detonated its first atomic device. Writes Francis Gavin, a professor at the Lyndon B. Johnson School of Public Affairs at the University of Texas at Austin, "It was predicted that India, Indonesia and Japan might follow."
At the time, he noted in a 2009 article in International Security, "A U.S. government document identified 'at least 11 nations (India, Japan, Israel, Sweden, West Germany, Italy, Canada, Czechoslovakia, East Germany, Rumania and Yugoslavia)' with the capacity to go nuclear, a number that would soon 'grow substantially' to include 'South Africa, the United Arab Republic, Spain, Brazil and Mexico.'" Mexico?
In recent decades, some countries have actually given up their nukes -- including Ukraine (which inherited them from the Soviet Union) and South Africa. Others, like Brazil and Sweden, have scrapped their weapons programs. After the Cold War, it was assumed the newly reunified Germany would want to assert its new status by joining the nuclear club. It has yet to exhibit a glimmer of interest.
A nuclear Iran would soon learn something previous nuclear powers already know: These weapons are not much use except to deter nuclear attack. What help have they been for the U.S. in Iraq or Afghanistan?
China invaded Vietnam in 1979 to force the enemy's withdrawal from Cambodia. The Vietnamese not only refused but sent the People's Liberation Army home with its tail between its legs. China regards Taiwan as part of its territory, but the island has remained functionally independent despite the threat of nuclear coercion.
If Iran does get nukes, its neighbors that have survived without them will find that nothing much has changed. Nuclear proliferation is the danger that lurks just over the horizon, and that's where it is likely to stay.