At least reddit still maintains old.reddit. Digg never gave users the choice. If there is ever only new reddit, then there won't be reddit for me.
The new mobile website is at least an order of magnitude heavier than the old one
There are some old screenshots of it, I've got one of them in my blog here: https://pdx.su/blog/2023-04-06-rip-reddit-compact/
I also assume they are smart enough to realize that some people will simply never use the app. In which case they're not realizing as much benefit from forcing the new UI on them. Sure, there's a cost to maintain the old one. But as long as things aren't breaking it's easier to keep it.