What do you think about deleting old blog posts?

Hey guys,
Some months ago I made an expensive mistake, since I've wasted some time and I haven't achieved some goals because of it.
But making mistakes is part of the process and it means that at least I have tried something. It can be frustrating at times but I need to accept it and learn from it.
Wondering what mistake I made? I wrote about topics that are not relevant anymore. Some of these topics brought traffic to my site for some weeks but people are not searching for them anymore, which means that my site has lost traffic.
That's why I'm wondering if I should delete the posts about these topics. If people are not looking for them anymore, why keep them? They are "wasted posts".
What about you? Have you ever deleted old blog posts? How has this affected your website?
I would appreciate your opinions on this topic.
Thank you =)
1. If it is no longer relevant at all, delete the page and 301 redirect the url to another more relevant page (or the homepage). I also remove it from Google via webmaster tools.
2. If it still has historical or archival interest, I will add a note at the top of the page dating the article whilst explaining why it is still there.
3. If there is a newer version of a product or if the page can be updated to make it more relevant, I will do just that (and maybe add an "updated: 2018" note).
If a page has a lot of "link juice" or people linking back to it, you don't want to lose that : )
Jerry