Similar to twitter.

If I go to twitter mainpage I can't see anything without logging in.

If I go direct to a profile I see can't see anything without logging and;

If I go to a specific post, I can read the initial post but nothing else.

That's what I expect will happen to Reddit

I suspect you're correct, although I doubt removing old.reddit will cause much of a stir.

Notice how Reddit is releasing 1 shitty update every few weeks rather than all at once? By the time one shit update is released the vast majority of users have forgotten the previous one. Remember the API change? I reckon most of Reddits existing users don't anymore. I suspect old.reddit will go the same way.

I'll bet cash that it's only a short matter of time before old.reddit is shutdown and within 6 months of that happening Reddit access will be locked behind account login.