Arguably you should factor in mining power too: if a cryptocurrency can be 51%-attacked easily (such as by an individual paying less than the cryptocurrency's market cap for servers in the cloud), then people shouldn't bring money near it.
You can easily 51% any and every coin for less than their market cap if you calculate just the cost of running/renting the hardware for a day. (Though really you don't even need it for that long)
I'm not sure if you would run into the problem of availability of hardware at some point, but if you're spending ~$76 billion to rent servers for a day, I'm sure you can figure out the details later.
But logically, 51% of the network cannot exceed the value of the network. You lose by a huge margin if you are spending more on protecting your money, than money you actually have.
What happens when you stop your 51% attack? The participants whose party you ruined can simply roll back the chain to before your attack and continue business as usual. 51% attacks are expensive because they must be sustained. Sure, you could try to take the coins and exchange them quickly, but if you attacked a coin with a big enough community (Bitcoin) the message would spread so fast that there would be a coordinated effort from stopping you cashing out. If someone just wanted to destroy a coin without caring to exchange for their currency of choice (like a government), well, even they would have to sustain a perpetual goose chase since people can just abandon the attacked chain and take up another and only provide support to the chain(s) that are not being attacked. It would take word of mouth, but we are using Internet where messages spread to millions with seconds.
If you make a longer chain than the rest of the network, then self-interested miners are going to switch to mining on top of your chain instead of risking their work on the old chain being for nothing. If the attack is short, then surely most miners won't be upgraded to be picky about chains in the time after the attack, and everyone else will know that and won't bother trying to make their miners prefer a losing chain.
If the point of the 51% attack is profit-oriented, then the attacker is going to do it long enough to get their double-spend confirmed and then stop doing the 51% attack.
Hmm, maybe market cap was the wrong comparison to make.
The cryptocurrencies listed in that article are doing pretty well for themselves mining-wise. I suspect that there may be tiny cryptocurrencies traded on some exchanges that could be 51%-attacked by an individual for somewhere between $1,000 and $100,000 to double-spend some high-value transactions. Maybe it's unlikely for the numbers to line up to be profitable and sure enough to pull off, but if it does happen, then that cryptocurrency is toast as soon as someone notices.
And, linked to that -- block creation rate. If a lot of miners exit a coin at once, it's possible for the hash rate and difficulty to diverge significantly, possibly even to the point that it may take days or weeks for a block to be mined. At that point, the coin is effectively dead.
Vulnerable, not dead. BTW even a 51 pct attack doesn't "kill" a coin, it just tricks users temporarily into honoring a double-spend. Networks can and do recover.
A 51% attack can kill people's confidence in a cryptocurrency, and finally wake traders up to the unsteady ground they're on. It's been a few years so I don't remember the specifics, but I remember seeing mentions of small cryptocurrencies that got 51% attacked, demand for it completely disappeared, most miners move off because it was no longer profitable, and no one at all kept mining because the difficulty value was left too high and had no time to adjust down to let individual miners mine easily.
If Bitcoin or a top-10 cryptocurrency got 51% attacked, I'm sure the price would take a hit but people would find a way to limit the damage. I'm mainly talking about tiny pump-and-dump-tier cryptocurrencies whose creators don't even love them any more but that inexplicably still have some trading volume.
