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There is a huge difference between forking the protocol with 51% hashrate and adhering to the protocol with 100X hashrate (both being ways to set a stratospheric difficulty level).
In the former case, honest miners will ignore the fork. In the latter case, they can't do anything.
ArticMine you are a physicist and I am a programmer for 37 years. I will be more skilled at my field and you will be more expert at yours.
The attacker in the 51% case is mining otherwise perfectly valid blocks but at a higher attack difficulty rather than the correct optimal difficulty in the blocks. There is no actual change in the protocol. The trade-off is that the 51% attack blocks could be easily detected by the network in exchange for a much lower cost of attack. In either case once the attack ends the difficulty starts to fall since time ticks by with no blocks being mined. There is a key difference between Bitcoin or a Bitcoin clone and Monero in that in Bitcoin the difficulty adjustment is discrete approximately every 2 weeks while in Monero it is continuous, This means that by timing the end of the attack correctly the attacker could keep the difficulty constant in Bitcoin for 2 weeks, while in Monero the difficulty starts to fall approximately 2 min after the end of the attack.
http://www.coindesk.com/data/bitcoin-mining-difficulty-time/http://www.coinwarz.com/difficulty-charts/monero-difficulty-chartCollusion between honest miners and devs could mitigate the attack but in both cases it involves ignoring the difficulty in the blocks in some way, effectively a hard fork. One can mitigate against this attack by having the difficulty algorithm drop the difficulty at a faster rate if no blocks are found after a given amount of time, where this time is statistically much larger than the normal block time.
Still I do not see the economics here. One can do a lot more damage with a 51% attack than this.