That ignores community consensus; such activity is easy to monitor and public sentiment about the illegitimacy of that fork can cause people to devalue it. This is essentially what happened with the DAO hack. This likely does create a situation where both forks have a non-zero value, but also dramatically lowers the rewards, and thus incentive, for such an attack.

Security in crypto is a very slippery concept, and many conclusions are non-obvious, if not outright counter-intuitive.