That's true to a degree; if someone were able to cripple half the world's economies just by disabling a cable then yes, that's a serious issue. On the other hand, if all the involved countries maintained backup power (i.e. gas or coal generation) that could be turned on in an emergency but otherwise unused, then the risks could be mitigated.

Backup generators would of course be expensive to build and maintain, but if they just sit idle 99% of the time maybe the cost would be acceptable. I assume a fossil fuel power plant that just sits idle could last a very long time.