The ideal of course is that Iran permanently foreswear nuclear weapons or any nuclear research and development at all, a not terribly realistic hope. But short of that, the real argument goes something like this: Even if Iran can't build a nuclear weapon for a decade or two, it will get an immediate influx of money and the ability to build a much more thriving economy freed from the weight of decades of sanctions and the especially onerous ones in place in the last few years. In addition, the regime itself will, shall we say, be koshered on the international stage.
For all the trouble Iran makes now for Israel and the Sunni Arab states it is still a pariah state, with no great power patron (though some measure of protection from Russia), limited ability to purchase weapons and an enfeebled economy. Changing all that is a loss for its regional rivals.
The best argument against the nuclear deal is that it is better to keep Iran as a pariah state, with an abiding mutual hostility with the world's great power, the United States, and still enfeebled by US sanctions, even at the risk of Iran building a handful of nuclear weapons. For the way many Saudis see the world this isn't only the best argument, it's actually a decent argument. For the Israelis, it's considerably more complex. But there's some logic to it there as well. I certainly do not blame the Israelis for vastly preferring a world where Iran has zero nuclear weapons - let's not forget: THAT'S WHAT THIS DEAL DOES. But Israel's generals do not think Iran will launch an unprovoked first strike against Israel to bring on the end times. That's not Israel's real problem. Setting aside the abiding issue of the Palestinian conflict, Israel's real problem is that Iran subsidizes and arms proxy armies to Israel north and south. The situation with Hamas and Gaza is considerably more complex than that. But with Hezbollah it very much captures the situation. Hezbollah is a vastly more lethal force than anything Hamas has or can do in Gaza. In any case, it is obviously a complex reality. But the point is simple enough: a legitimized and wealthier Iran which does not change its behavior presents real challenges to all its neighbors.