I'll try and tl;dr what happened before and after Hive was born and how it works.

Basically what happened with Sun was pretty unfathomable, he acquired through a deal with ned all of Steemit's stake which was quite a lot and there's no such stake available anymore in a single person/entity (all was transferred to the DHF @hive.fund after the fork). With this stake alone he managed to get some of his fake witnesses in the top20, then the community, surprised by this, started rallying everyone to move their witness votes from those outside of the top40-50 onto those with a chance to get back into the top20 to regain control and governance of the chain. 17 out of 20 accounts are required to push a hardfork through. It was going well for a while, real genuine witnesses were filling up the top20 again after everyone was getting in contact with everyone else, powering up some steem, etc.

Then Sun decided to be a sore loser after losing majority control of his fake witnesses and used his contacts with exchanges to "trick" them that there was a security issue and that they had to power up all of their customer funds and proxy vote for his accounts. With that he managed to get control back over the network and we decided to instead fork over. There were other stakeholders at the time who also voted for Sun's fake witnesses, them along with any other accounts that voted for more than 2-3 fake witnesses were left out of the Hive airdrop, everyone else got the same amount of hive and hbd as they had steem + sbd.

Now with Hive we've implemented a rule where when you power up Hive it takes 30 days for that hive to take effect on your witness votes. For instance if u have 500m HP and power up another 500m HP, your 2nd 500m aren't going to affect the witness rankings until after 30 days of the power up. This is to give us time to act if we see an attempt at such an attack again the way exchanges did back then.

Then of course they could also try a "sneakier way" where they power up little by little over a longer period of time, but eventually this would still show on the witness list. People would question who this new witness is that's making his way into the top20, start checking who's voting for him, who else those accounts vote for, what they do, what the witness does, "does anyone know him?" etc. Similarly as back then they could start rallying votes to unite the community to temporarily vote on the real and genuine witnesses to make it harder for this new unknown one('s) to remain in the top.

This is of course quite hypothetical too, we don't know how much of the liquid Hive on exchanges belongs to someone, how much belongs to active hivers here, how much to random investors big and small, etc. In the event they'd try and do a quick attack we'd see it in the price fluctuations as it happened back then when Sun and the community were powering up Steem to regain access of the network, it went from like 15-20c to 80c in a short period. If they were to do it slower it'd go unnoticed on the market side but we'd be able to notice the power ups and things over time and question what these accounts are and what their motives may be.

You can see by the witness page for instance, @blocktrades is at rank 1 right now with almost 50% approval. This means that of all the powered up Hive right now almost 50% of it is voting for the account. Last in the top20 has 38% approval %, so for someone to compete there, even now at times of "peace", they'd have to power up at least 60-70m hive. While there's quite a bit of liquid Hive on the market, I believe about 50% of the total supply, so about 200m, we'd still be able to notice such big moves in time and with the 1 month governance vote delay act accordingly.

In the absolute worst case scenario that someone would manage to overthrow governance again and say power up 100-200m Hive, we can always just rinse and repeat what we did on Steem. We'd move over to a new chain, all dapps and services along with the community, we'd airdrop to everyone aside from those who voted for the witnesses that overtook governance, trade all of our Hive for new coin, thank the attacker for the profits we made along the way (trading hive for new coin, of which we already got an exact same amount of as we had Hive), put a ton of sell pressure on the old coin (hive) which the attacker has most of the supply of now and leave him with a dead community, no dapps, activity and a falling price as the other one does better.

Hope that helped get some clarity on the whole thing and why Hive and the community here is so robust and why it's so difficult, expensive and pretty much futile to attempt to overtake our governance.