Pinterest is already deeply demoted.
I'm using Pinterest results as a sign that there is literally nothing else that matches my search.
Part of search quality is serving nothing when there are no results. Putting a bunch of irrelevant Pinterest pages in the results, just because there are no good results, isn’t good for users.
I don't agree. I may be desperate for a result in some situations. If there's a small chance there's something useful out there, I still want to see it. I get to decide whether it's worth my time to look.
The problem with Pinterest is that often the image looks exactly like what you want - eg search for diy furniture plans and a lot of links are Pinterest. That image came from a source somewhere, but Pinterest has better SEO than the source.