That version of reddit was created in 2005. The world has changed a lot since, and users are expecting certain features. If you don't have them, you are going to struggle to grow nowadays. For instance, the majority of users are on mobile now, so you need to offer them a proper UX.
It isn't that simple, for example I don't use old.reddit as a URL, but I use RES or another program to force it into the old configuration regardless of the URL.
