Right now I'm not seeing any of these problems that OP is talking about when using old.reddit either. I think I may have ublocked all the banners and popups at some point.
It's impressive how much engineering effort reddit is able to waste, that isn't cheap.
As for new users, it's likely a lot of people that reach Reddit from Google - their SEO seems to have gotten loads better in recent years. They also don't remember how simple oldreddit was, so they have no reference point.
I do sorta wish HN had reply notifications.
There is a 3rd party app for that, though I haven't tried it yet.
The only problem with it is that it does not send you replies to your submissions, e.g. AskHN posts.
It kinda weird because for some reason in my mind Ycombinator was 5 year-ish older.
AAAAAND you lost literally 95% of every single person who might even consider using this. Stop trying to make "fediverse" happen. It's not going to happen. All you do is end up putting great developers into projects that will literally never, ever amount to anything due to an insistence on decentralization.
The federated alternative that I'm building[1], is trying to specifically keep some of the benefits of old reddit (and of HN, and of lobste.rs), most important of which being that it has no reliance on Javascript and that it has a minimal size footprint.
[1] https://littr.me (just a demo instance)
P.S.
I suspect that one of the issues with Reddit is a large number of regular visitors who lurk. No account, no login, never post, never upvote... but spend hours a day on the site.
Serving them means that features for actual active contributors have low priority. I suspect that looking at the site and its features with that in mind may explain a lot.
They would be shooting themselves in the foot because the lurkers are looking for posts and comments. Prioritizing contributors would keep lurkers engaged longer, watching more ads.
Also, never underestimate the need of people to voice their opinions. Controversial topics will tempt lurkers to contribute.