Your question seems to me to imply some level of mutual exclusivity between rpg and web programming and this is not the case. I also wonder why one would want to pose this type of a question based upon technology (such as rpg or java or whatever) instead of business capabilities or functionality.
We are doing a lot of java for web programming in a apache and tomcat and struts and tiles environment. However, our data base is iSeries and our business logic is in rpg. All of our stored procedures, triggers, and batch programming is in rpg. We use rpg in conjuction with C and some java (eg we use the javamail from rpg). So, the answer to your question about technology is to use all of them to the best advantage of each.
Also, you imply that rpg is old technology. I do not think this is the case. Now, the green screen interfact is old techology but this has little to do with the underlying programming language. With the advances in rpg/le recently, you might want to reconsider your basic question.