SEO is black magic and astrology, so there's a lot of guessing going around, but I think I can vouch for this as this particular issue happened to one of the sites I maintain, and it caused the homepage to lose lots of traffic because it was bundled as related with some ancient blog posts. the homepage performed quite well according to all the metrics, but old pages did not, and until Google started penalising groups last year it didn't make a difference to keep the old pages around. Since they started penalising random groups, something that's important can be affected by something old and irrelevant, and the feedback loop is very slow.
It takes 28 days to get feedback from the web console about any potential changes to page performance, so I tried fixing the old pages in a few ways, then just gave up and deleted them. the average rose up, the penalty to the whole group was gone, and the homepage again started getting a decent amount of traffic.