This seems to come up a lot. Your definition of rational is a short term rationality only. I can pass up a short term profit in return for more stable longer term profits and be completely rational, by a reasonable definition of the word.

I think it's clear by now that if most or even some miners decide to prioritise short term profit over the long term health of the system (i.e. longer term profit), Bitcoin basically doesn't work right. This attack is only one of several such things that can happen. This certainly can be a problem when difficulty is skyrocketing because a mining investment is I guess quite short term anyway, but presumably at some point the mining arms race will end and miners will become more settled in.