My treadle is a Damascus and it belonged to my Great-Aunt May. Before it was given to me I was looking at various treadles because I wanted to sew with it. This was before the internet but I wanted to have a pretty piece and I wanted to be able to use it. So I was looking for one with the book, any attachments and I could test. When I told my grandmother I was looking she asked me why I would want to buy one when she had 2 of them. Luckily for me, my family has always been the type to never throw anything away so the book for the Damascus was still there along with a few attachments. (It's from 1925.) She also had a Singer, which is quite a bit older, but no book and had been used by a neighbor from time to time to sew leather items.
As for letting other machines go - wow, that's a hard one. I had three 221's and thought it was a waste, so gave one to my sister and the other to my niece. (They sew and neither of my DD's sew.) Hopefully you can gift them to someone that will love and take care of them as you have.