Right now, 100% of my usage is old.reddit.com and 0% of my usage is on the new design. These numbers won’t change if they remove old reddit — I’ll just stop using the site.
I keep hearing this repeated by people who use the site. Honestly, no they won't. At least the vast majority of them. There's literally no viable replacement.
For tech people, you're looking at the viable replacement. For other people, if they're deliberately going out of their way to avoid the redesign after all these years, they may replace it with nothing (and be happier, as most people find they are when they quit social media).
old.reddit.com may not be long for this world. i.reddit.com and reddit.com/.compact were both removed earlier this year: https://news.ycombinator.com/item?id=35283379
(The irony now of that top comment, "99.5% of my usage is through the iOS client Apollo"...)