Netanyahu argues that if the U.S. walked away, Iran would eventually capitulate on everything; the “better deal” he imagined is one in which Iran does everything short of dismantling its government. He had nothing to say about why this might happen if we weren’t negotiating, other than that we should “keep up the pressure.” That’s his alternative: Do nothing, and instead of just going ahead and developing nuclear weapons, Iran will see the light and completely reverse everything it’s been doing.
To call that position “absurd” is too kind. You don’t have to be some kind of foreign policy whiz to grasp that there’s something weird about arguing that 1) Iran is a nation run by genocidal maniacs; 2) they want nuclear weapons so they can annihilate Israel; and 3) the best way to stop this is to abandon negotiations to limit their nuclear program and just wait to see what they do. But that’s the position Netanyahu and his supporters in the Republican Party are now committed to.