The ROI of attacking the network goes up as the price goes up if the hash rate stays stable.
Which it shouldnt, because more miners should come if the "miners tax" stays stable...
If the miners tax stays fixed it could make an accordion effect on miners/hash power/price but that changes once the miners have invest a lot of money and they have a huge fix cost, so they have to stay in market yes or yes just waiting for better times...
Then they will have to ask to rise in the miners tax.
why do miners prefer to mine in pools? they have very good reason: the volatility of their income is terribly impractically huge if they mine alone, even though the average revenue is theoretically the same (if neglecting some more factors). so if you have an ability to keep the expected average profit but significantly reduce volatility, that's what what rational players usually do.
same applies wrt the volatility of the price.
but risk-free concept sounds quite strange for me.
"risk-free" investments are bonds, which in my opinion is not true anymore.
There is always risk associated to the project. (different points of view in the community/team, the leader leaving the project)
imagine we have an agrs/btc options market and also a spot market (i.e. being able to convert agrs to btc and vice versa). then if one sells (shorts) a call option, and in the same time buys (long) the corresponding put option, and also buys the underlying assets (all with matching amounts), you'll have a profit for sure (neglecting commissions and other frictions). this profit should theoretically be equal to the so-called "risk-free interest" in the market, but more precisely, it really is the "alternative cost", i.e. how much people believe they'll profit if their money was not locked for a given amount of time. this technique is as old as black&scholes model