I never access /r/popular or /r/all or any of the default subreddits, all my usage on reddit is from niche communities about my hobbies, and through the past 15 years each new hobby I started there was a subreddit to kickstart it full of information and a community. That's going to be the biggest loss.
Most of reddit the past 10 years has been shifting towards new users just lazily scrolling /r/popular but those users are not profitable, they don't generate value, the ads will be poorly targeted. The laser-focused subreddits is where the value of reddit as a platform comes from, I don't like advertising but I could tolerate targeting on those subreddits as it'd be relevant without invading users' privacy (like the old days of AdSense using the context of the page for ads, not profiling the user).
The vast majority is utter stupidity, I agree, but that is a part of reddit that you can completely avoid if you are a bit more of a power user... It will suck when it dies.
I've read about some people calling for a return to forums. That lacks the one-stop-shop-for-all-interests feel, though - besides, subreddits are quick to tell you about other online communities you should hit up. For some of the older folks on here, how were the old newsgroups compared to reddit? From the little I know - and I mean "I learned about newsgroups from a joke on the Simpsons" little - it kind of sounds like Reddit...