[newsbinator]

Is there much to maintain on old.reddit.com?

It lacks all the features that make new Reddit annoying and slow, and it doesn't need to gain any new ones.

It's basically a graphical version of the core api, which is solid.

[Trasmatta]

The cost is probably relatively low (but not negligible). I could imagine a future though where they decide to make some big breaking changes to the API, and decide it's not worth updating the old site.