The UX redesign was hugely successful in attracting new users, users who could then be monetized.
The "useless answers" are wildly popular responses because people generally prefer to meme, not solve problems.
Your complaint essentially boils down to, "Why do people not behave how I want them to?" and that, my friend, is a question as old as time itself.
From the HN guidelines, but it also applies to Reddit:
> Please don't post comments saying that HN is turning into Reddit. It's a semi-noob[0] illusion[1], as[2] old[3] as[4] the[5] hills[6].
[0] https://news.ycombinator.com/item?id=926703
[1] https://news.ycombinator.com/item?id=633099
[2] https://news.ycombinator.com/item?id=582513
[3] https://news.ycombinator.com/item?id=289254
[4] https://news.ycombinator.com/item?id=253657