Features can be added without alienating users with outdated tech. Just conditionally show the improved site only to users who can run it. It's a poor excuse, especially since Craigslist hasn't even tried to improve the layout of the site, which has no effect on load speed and compatibility.
The same is true of Facebook. That hasn't stopped them from evolving their design.
I've seen padmapper slow wayyyy down on my i7 macbook with 8GB ram, I can't imagine what it'd be like on grandma's 5 year old budget PC.