I have noticed this at some of the sites I used to write for. Hence, as soon as I put a new piece up, I archive it at archive.is, and include that reference in my site's list of my work. Periodically, I should go there and check each one, but there's a lot of material.
I did not know that this was a possible motivation as to why my more 'historic' work is disappearing, though.
Sites also just reorganize, change CMSs, simply go out of business (which can happen to archive.is as well). I save a lot of my own stuff but it's a bit hit or miss and I expect a lot of the material we lean on The Wayback Machine to save is probably pretty hard to actually discover.
True about archive.is being probably more at risk than WayBack Machine. I just don't know when I'll ever get the 2-3 days necessary to additionally back the posts up at WM, because each save is very slow.
I did not know that this was a possible motivation as to why my more 'historic' work is disappearing, though.