A big proportion of an insurance premium is to cover the risk of having to pay compensation for personal injury or death, which can run into the millions. Whereas when I was getting quotes for a 14 year old Yaris, worth at most £2000 some insurers quoted the same price for comprehensive cover as they did for third party fire and theft ,as their additional risk would only be writing off £2000 .
Its understandable that as they now risk having to pay £30,000 to replace a new car rather than £20,000 value for a 3 year old car it may increase the premium ,especially as some insurers companies will replace new for old during its first year or so. But the actuarial increase in risk may not be clear cut so its still worth 'shopping around' for a better deal.