It's a design we've modelled and predicted should work. Since we've never actually built anything close to it, we have no way of knowing whether those models were even close. If ITER shows results that are even close to breakeven you can expect a rush of newer designs using technological advances to shrink scale and improve performance.
Besides, ITER is also meant to help develop experience in handling large plasmas for extended periods of time, that knowledge will transfer to other designs.
ARC actually is a pulsed design, they design against handling plasmas for long periods of time and actually using shorter duration is a net win for efficiency and reliability.
Also, ITER won't demonstrate anything until 2035. By then, several smaller scale designs would have already demonstrated break even.
Besides, ITER is also meant to help develop experience in handling large plasmas for extended periods of time, that knowledge will transfer to other designs.