I keep all my bookmarks in a Delicious account and I was shocked that around 40% of links I saved since 2013 are now gone. I managed to find few archived pages in archive.org but many of the pages I "saved" are gone forever and I can never refer to them again.
I now save every single page I find important or interesting on my local HDD and then move it to a backup disk later on.
I was having the same problem with my bookmarks disappearing from the internet. Searching for a solution I eventually made a browser extension to send bookmarks to an online archiving service (archive.is). I then added an option to save archives locally in mhtml format. I have around 370 bookmarks archived that way and it takes 240MB of space.
Saving archives locally is great, but I wouldn't count on using archive.is for long-term reliable storage. Seeing as how it must cost its operator(s) more than a few ducats to operate and stores copies of copyrighted material on a centralized server(s) without being able to hide behind the "lol we're just a library" shield that Internet Archive can, I wouldn't be surprised to see it suddenly disappear one day.
Your comment reminded me that I have a Delicious account, which I haven't used for years. I logged in and just for fun went straight to my oldest link (saved in September 2006), and it still works!
I now save every single page I find important or interesting on my local HDD and then move it to a backup disk later on.