This is the "inevitable disclosure" argument - AKA the idea that the experience is the secret, and thus nobody should ever be allowed to switch employers ever again.

For various reasons (notably, the fact that slavery is illegal), we don't accept this in general. You have to show that secrets were copied in full. Employees cannot memorize millions of lines of source, they can only memorize vague architectural details that would be easily reverse engineered by competitors. If you want to own those vague details, get a patent, or shut up.