Just as one example I can recall, you can do things that are kind of like "quote retweets" with the cross-post feature that don't really make sense on the old site (on "new Reddit" when you crosspost the original post/title are displayed in a little window so you'll see people make titles that are actually commenting on the original post and not on whatever the link actually is). On old Reddit these just show up as links to whatever the original post linked to, but with the "meta-commentary" title. More than once I've encountered one of these with confusion until a comment explained, or I thought to check the tab that shows where else the same link has been posted.
Same. And many of them message me directly via chat when they have a question about a subreddit I moderate. Chat which I check about every two months, which is for some reason fully distinct from both the regular messaging functionality (which to be fair, is very well hidden) and the two methods of contacting the moderators.
old.reddit.com + RES is still the best Reddit experience.
That said, I agree with you in principal. In this field I feel like I'm constantly fighting to keep the code simple and lean, while others think it's okay to import an entire bloated library just to make a button shine a certain way.