It is about developing and maintaining applications that are robust and flexible enough to embrace the latest technologies. To accomplish this task, you could rewrite your applications in the latest trendy programming language, but your development cycle will be long, and you will need to retrain your current development team or hire a completely new one. There is no guarantee that the new programs' functionality will be the same. You know your applications will continue to perform as they always have. What you need to know is that those applications can co-exist and function with other technologies.
Cost is an important factor in such a decision.
Obviously, rehosting makes financial sense.