Hate to break it to you, but nobody uses old anymore. As a subreddit mod you can see traffic breakdown stats, and old makes up around 5% of traffic. Up to 10% on some subs. I'm a moderator of a highly technical niche sub and it's about 5-10% for me. Other mods of various subs have commented with their stats and it's always in that 5-10 range.

Old users may create more content than normal users, I don't know about that. The niche subs might take a hit, but the main website and big subs will continue on without disruption if they kill old. (Assuming mods continue to mod - and Reddit can replace/hire mods as needed)