I've got some simple server code that uses the String class and had the server respond to a client request 500,000+ times with out a crash. Perhaps your code is defective.
If you are very cautious and know where the class is defective for en embedded environment, this is possible. But generally it's probably the worst decision of the Arduino makers to have this class included in the IDE. The way the OP uses the class he won't be able to answer 500'000 requests. And he does nothing the documentation says you shouldn't do.