They could have spent the time they used to produce the current mess to learn about the old code and improve on it, or document it and then use it as a starting point for their new stuff and then improve on it.
10 years isn't necessarily bad either. Just look at HN. Functional with minimum of JS, 2.0kB transferred to load this reply page, tho it will not win any fancy design awards anytime soon. But all in all, great user experience. If you wanted to inline the reply form into the comments pages, that can be done easily in under 1KB of unminified js.
Loading the reddit frontpage (not signed in, adblocker): 21.8MB transferred (11.7 without media and fonts), 501 requests (183), 20+ seconds to load, 11667 ms of scripting. What the actual fuck.
old.reddit.com: 2.7MB transferred (2.1MB), 100 requests (24), <2seconds to load, <300ms of scripting.
