I second everything he said. Nothing is ever entirely secure, and I don’t believe this is a reason for abandoning WordPress.

Regurlarly updating old versions of software platforms and formats is almost always the best road to go in these things. When an update is due, and your theme doesn’t support the new update, this shouldn’t mean you shouldn’t update the platform. Site-designs in general profit from regular updating, so simply update your design too when you’re in need of a platform update.

I would however advise to update when you have some spare time on your hands, since checking if everything works can be time-consuming.