I am or rather was one of those awful librarians that threw away hundred linear meters of books. I personally had many books until we came living here and I had to separate from 3/4 of them. As a former librarian I understand that the risk if you keep buying books without getting any out isn't that the shelves will topple down, but that the floor will. It's a main concern when calculating how many books can be kept in a library and it should also be in an old house.
Any that were of interest found a home at my parent's place. My mum is slowly reading them and every now and then she wants to discuss some point of detail in a book I don't remember I ever owned or read.
But I still have doubles, many in fact. One reason is that I met my partner in our late thirties and he also had a lot of books, and while we have very different tastes, there was a number of books we both owned and wanted to keep. Another reason is language. Some american authors I first bought translated in french and then I bought their books in english because reading in the original language is so much better.
I love e-books but I don't think their use is exactly the same as physical books. I would never attempt to read essays, poetry, or comics in e-books form.
My mum's eyes are bad enough that if there is an e-book version she will choose it for the possibility to zoom, and she still always takes manual notes, on notebooks dedicated to that purpose. It's pretty efficient, she always makes a small synopsis, writes her opinion of the book, and takes quotes noting pages number, on a double page. I wish I could be that organised !
I find it really is a confort when you read in your bed at night to have a very light device compared to a heavy thousand page book, and one that can be dimmed enough so as not to bother someone else trying to sleep in the same bed

.