Agree with @neo but also worth noting the other difference you highlighted:
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51% attack on blockchain means control of the entire network. That means you could very subtly choose exactly which data you manipulate and when. So the scope of such an attack is enormous, including of course bringing down the whole network. It would better be called the God attack because it’s power is total. You can identify individual wallets and possibly individuals and target them, and in all likelihood few people would ever know. This immense power and versatility makes such control very attractive for a wide range of purposes, criminal, commercial and political, which in turn makes them more likely to be attempted.
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controlling a section on SAFEnetwork would mean that for the time you have control, you could do at least these things (I think): a) block actions that happen to data controlled by your section (get, put, transfer SafeCoin etc), but only in the tiny amount/fraction of data your section controls, not the whole network and you can’t target individuals or content because you can’t identify the individuals or decrypt the data you are storing. b) delete data that is being looked after by your section. This is more serious than a), but again the scope is limited to a tiny fraction of the network storage, and again you can’t choose which data and you can’t target individuals or particular content. c) steal a SafeCoin whenever one of the SafeCoin in your section is being transferred by intercepting the transaction and modifying it. I don’t think can just go ahead and steal all the SafeCoin in your section, at least not easily, but if you can, it is still only an extremely tiny fraction of all SafeCoin, and again you can’t target it - the theft will be random.
Even though you can do much less, controlling a section is still very hard, similar scales of difficulty in terms of cost of resources than a 51% attack once SAFEnetwork passes a certain size. And much more difficult through centralised political or commercial control (whereas the number of miners needed to control 51% of bitcoin is quite small, making it very vulnerable to this kind of attack IMO).
But the big difference is that the rewards for the attacks on a SAFEnetwork section are tiny whether your aim is sabotage or theft, and this matters.
This is a great protection, because attacks are less likely if the rewards are less, or simply don’t justify the cost of attempting them - especially if there are more profitable targets, in which case you can pretty much rule out attacks motivated by theft.
So on SAFEnetwork we may only need to worry about sabotage for political or censorship reasons, because there will almost always be much more profitable targets for thieves. The good news is that if SAFEnetwork succeeds in decentralisation as intended, these attacks become very difficult and impractical once the network passes a certain size.