If `old.reddit.com` goes down forever I will be very disappoint. New Reddit doesn't work with JS disabled, and I enjoy surfing with JS disabled because I enjoy the privacy benefits.
I hate to be the party pooper but I think I know when they'll do it. Reddit has been desparate to IPO for years now. They're not going to do anything to shit on engagement in the run up to that, but 6 months post-IPO I think it's innevitable that there's going to be a discussion along the lines of "Why are we torturing our devs maintaining this old interface we can't monetize" and they'll finally can it. Obviously anyone who actually contributes to reddit is using old.reddit and so the quality will go off a cliff, and reddit will slowly slip down the toilet, but only after they've dumped it on the open markets.