Something very interesting happens soon.

Following automatic difficulty reductions, mining a Bitcoin Cash block is only one quarter the cost of mining a Bitcoin Settlement block. Bitcoin Cash is already 15% of the price of Bitcoin Settlement. If the price hits 27% of the Settlement price, it will turn the tables and Bitcoin Cash will become more profitable to mine.

To understand the magnitude of that, you’ve got to remember that most miners will mine whatever is most short-term profitable. There is some principled, or contracted and inertial mining. But miners with millions of dollars of equipment, a.k.a. miners who matter, mine for the highest short-term profit and turn on a dime to do it.

In addition, mining is an all or nothing game. Many intuit that Bitcoin Cash has 20% support or 20% market price, so it will secure 20% mining power. But it doesn’t work like that. Either Bitcoin Settlement is more profitable to mine and it approaches 100% of the mining power or Bitcoin Cash does. There is some inertia, as described above, but generally, all the mining power flows to whatever has the maximum expected value for the next block.

Pretty soon, that’s going to be Bitcoin Cash. Now consider what happens when miners start mining Bitcoin Cash in earnest.

First look what happens to Bitcoin Settlement. It loses miners. Blocks start to slow down as miners chase the gains on Bitcoin Cash. Bitcoin Settlement was slow and congested before with 6 blocks an hour, wait until there are only three block an hour, or two, or one. I don’t know if the Core developers were correct in the assertion that spam was driving up transactions costs, but if it was, expect plenty more spam. It is a low cost attack for Bitcoin Cash’s supporters. There will be lots of transactions too as Bitcoiners try to switch into Cash. This causes a vicious cycle of a clogged network -> lower prices -> less mining -> clogged network.

Meanwhile Bitcoin Cash is whizzing along with fast blocks, low transaction fees and rising prices in a virtuous cycle. More hashpower -> more confidence -> rising prices -> more hashpower. The mining difficulty only resets upwards after about 1800 blocks or about 3/4 days of this. By then the price may have increased sufficiently for it to be more profitable for miners to stick with Bitcoin Cash. Bitcoin Settlement will then need an emergency hard-fork to get restarted.