Not everything newer than COBOL is a fad. Python or Java will still be written many years from now. Maybe not 50 years from now but some languages will not go away anytime soon.
I think Java is in group that is used in 50 years. Python might fall way of perl, php and others... Specially if there will be 4.0...
Only threat to Java is the success of the JVM, that Java could be replaced by another JVM language seamlessly, but the JVM itself will definitely be here and developed 50 years from now.
Say what you will about its merits, but PHP is used by over 75% of websites today[1]. Rumors of its demise are monumentally exaggerated.