I wonder if Graham was presented with the same problem today, will he pick a lisp and write everything from scratch or pick up a less "expressive" language because it has a strong and mature ecosystem of libs.
The reason Paul Graham used Lisp is it was the only reasonable way at the time to implement RTML, which was the HTML macro language with online editor that Viaweb was built out of. RTML was how every website in Viaweb was constructed, it is what allowed them to move quickly and customers to customize with unequaled flexibility.
It was their key competitive advantage and Lisp was critical to its creation and function. It would be foolish to attempt to implement RTML in something else. Perhaps slightly less foolish in 2015 but not by much.
You can have your cake and eat it. It's possible for Lisps to use library code in less expressive languages, such as C, via a variety of mechanisms (foreign function interface, extending the virtual machine, ...). If you want access to Java libraries, you can use Kawa, Clojure, or Armed Bear Common Lisp.
There's a solid trend to do that, Norvig wrote a lot of recent articles in python, colleges are replacing Java with it too. Lisp genes have spread so it's not as alternative as it used to be. That said, SBCL seems to have possibilities and performance unmatched by all the tiny trendy languages of the day (except for v8 maybe and lua maybe).
At the time when PG was solving a web development problem, web development problems were "new and interesting", so it made sense to use a Lisp.
Nowadays it probably doesn't make sense to use Lisps for web, but it would absolutely make sense if you are solving something that is actually a new and interesting problem nowadays...
It was their key competitive advantage and Lisp was critical to its creation and function. It would be foolish to attempt to implement RTML in something else. Perhaps slightly less foolish in 2015 but not by much.