When the existing code base is at least functional, then of course, a big rewrite is not warranted.

However, some of us are not as lucky. The situation is often such that the existing code base is full of bugs that it can cause a ton of support requests from the users, or even crash and bring down the production server.

In such situation, you can choose to firefight all days and have no time left to implement new stuffs, or choose to rewrite.

Not using the latest hippiest libraries is really the least of my problem.