Now, let's look at the comparative costs of losing some typical pirate ships versus trader ships. We'll go with the worst case: total loss. Blown up in the fight. Anything less than a total loss is still a comparable and proportional difference between the two ships, since repair costs scale with the total cost of the ship. Note that "trading" ships also have to factor in the cost of rebuying lost cargo...
...As you can see, the COST is much higher for the typical "trader" prey versus the "pirate" predator. The typical pirate will need only 5 to 14 minutes of trading activity to recoup a _total loss_. Meanwhile, the typical ships that traders are flying will range from 77 minutes to 199 minutes to recoup a total loss. And the spread is even worse for T9s and Anacondas.
Ok, I finally understand the point you're making. If traders and pirates were different 'classes', and you could only play as one or the other, Elite's current system would be fair enough - but instead, a pirate who gets blown up can simply jump in a Type 6 and do a trade run or two, and he's made his money back (just like the pirates in Archeage, who only had to work for 5 minutes). A trader who gets blown up, on the other hand, has to spend over an hour trading to try and make his money back (just like the traders in Archeage, spending 2 hours). So, the game is unfair.
However, as you say yourself, this is a worst-case scenario. According to Frontier, piracy is not meant to be about destroying traders at all - it is simply supposed to be a 'stand and deliver' sort of thing, where the trader will live to tell the tale. In order for this to actually be the case in-game, there needs to be a far heavier penalty for player-killing, to discourage pirates from murdering their victims... but if such a thing IS introduced, traders will almost never get destroyed, so the high insurance costs that you described will rarely be an issue. Pirates, on the other hand, live a more dangerous life, and can expect to get destroyed fairly regularly (by bounty hunters, police, traders with good defenses or wingmen, or even by other pirates) - each individual death may not cost them very much, but repeated deaths will start to add up, and over time a pirate may end up paying the same (or more) in insurance as a trader will, making the game relatively fair.
You also mention that even if a trader is not destroyed, repairing damage can be very expensive on the bigger ships... but I think Frontier did this deliberately, to encourage traders not to fight at all. If a pirate interdicts a Type 9, the trader can simply submit to the interdiction, give the pirate a few tons of cargo, and then fly away without taking any damage at all... the pirate is happy (because he got what he wanted without expending any ammunition), and the trader is happy (because he will still make a 250k profit anyway, a few tons of cargo is practically nothing to a Type 9). This is good design in my opinion, and it shows that Frontier don't just want pirates to let traders live, they also want traders to let pirates live as well, instead of always fighting to the death. And the loss of cargo that traders suffer as a result is (or should be) comparable to the fines and bounties that the pirate incurs, so again, it's approximately fair.
This whole debate was prompted by your concerns over Frontier's intention to adjust the way interdictions work, by making it more difficult for traders to escape quickly - but in the end, even this may lead to fewer traders being destroyed, not more. As it stands, a pirate must open fire on a trader immediately, before the trader escapes - there is no way to simply request cargo, instead you always have to fight. But if it took longer for your FSD to cool down after an interdiction, the pirate would have more time to 'negotiate' with the trader - and the trader would be more inclined to listen, instead of trying to run, because escape would be more risky. On the whole, it would lead to more peaceful encounters between traders and pirates, which would only cost the trader at most a few tens of thousands of credits... making the game less frustrating for the traders, and less frustrating for the pirates as well!
Of course, all of this rests on Frontier introducing a suitably harsh punishment for player-killers... but as longs as they do, I see no reason to worry about their intentions for the interdiction fix, and I certainly don't think it is necessary to make interdictions balanced - because the game
as a whole will be balanced, and that is what matters most.