How often do you come across a site that installs malware? I've never understood why people limit their experiences because something could happen and that could is rare. I'm not going to drive because I could get in a car crash and die. I'm not going to eat certain foods because I could get sick and die. Now if the chance is 9 out of 10 times then that is a different story. But if it is 1 out of 100,000 I'm not going to worry about it.
Being realistic, phpBB4 won't be released for many years. We don't know what the browser technology will be and what people will expect from web sites. No one can properly discuss what the user experience will be at that time. It is like betting on who is going to win the world cup 5 years from now. No one knows.