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.