Looking at one of the subs I run though - old.reddit.com accounts for 5% of total traffic. Whilst 5% is not nothing, it's also not really all that much. I doubt they'd notice much impact even if half of the old.reddit.com users left.
Don't get me wrong, the old UI was awful, but it was much more information-dense, so I prefer it. I think it's unlikely that anyone that's not already a turbo-nerd would come as a completely new Reddit user, compare the two, and pick the old UI.