The aim and purpose of Reddit is a combined link-sharer and discussion forum. It is very like Usenet or web forums of old. This purpose is captured perfectly in the design of Old Reddit and practically eschewed by the New Reddit.
New Reddit is clearly intented for consumption, not creation. This is not built on the bedrock of Reddit's strengths.
Old reddit embodies the spirit of those old sites. It attracts users who value that spirit. This is what makes it enjoyable and for many, 'the internet's front page'.
I hope Reddit never make thier new layout manadatory. The website's strengths derive from the opposite of everything the new layout suggests; it is a site for users and not another 'social media' page. There's no need to make such breaking changes that make the site unrecognisable; consider Hacker News, 4Chan and how long they have kept the same simple-but-functional layouts. This is good web design when the focus is on discussion and link or image sharing; the layout itself isn't distracting to the user. Because it doesn't clamour to be noticed, the layout becomes a beloved part of the website due to its many years of faithful service. Rapidly tearing it apart like a hearless city planer does to a historic building makes a familiar place unrecognisable and thus less attachment, and then less incentive to visit. Remember Digg.
I'm aware of alternative old-Reddit styled communities, but these attract fringe groups disproportinately. Reddit represents a website where people of all stripes and opinions congregate.
One can raise complaints about unpopular opinions being buried and heavy-handed moderation, though on the whole there are 'subreddits' for most areas of interests where opinions are open to challenge. This level of community and collaboration that includes many disparate groups is difficult to recreate. I should wonder if anywhere else will create quite the same community were Reddit ever to disappear.
I'd actually suggest Usenet over Reddit generally, being one who prefers protocols to platforms. I know that a resurgence of a threaded message exchange protocol, open to anyone who wishes to set up a server and write a client, is very unlikely. But it would be a better realisation of the open spirit of the early Internet.
The new Reddit layout is a case of having something that is almost-perfect and then seeking to exceed that with something new, only to forget everything that made the original attractive and thus it falls far short of the original.