Serving different content from the same URL is generally a bad thing.
No, it’s not. That’s the reason they invented Language-headers in the first place: So you can view a page in your language and I can view a site in my language. Please respect that not everybody can read english
I think the context was that language headers are one way to solve that problem, but with hindsight they are inferior to the solution of the language being in the URL.
Caching is a very important practical reason. Just as important, for me at least, is that if the preferred language is part of the URL I can forward a URL to someone and be sure we are looking at the same content.
Generic pages that use the accept-language preferences to redirect to the correct page are one possible way to please everyone. E.g. wikidata.org could redirect to en.wikidata.org or de.wikidata.org or ja.wikidata.org. But those redirects create extra load/bandwidth, and it might be too late for it now (i.e. if millions of wikidata.org URLs are out there in the wild).
Darren