I used to always finish books once I had started reading them,
unless a book was really, really bad. I can think of only two in
decades past that I started but did not finish.

I reasoned that I had either spent money or at the very least
invested some time on the damn thing, I may as well see what I can
get out of it. A perfect example of the sunk-cost fallacy. Nowadays,
I am far more protective of my time. I give most books a few
chapters - if they don't grab me, or make me want to keep reading, I
ditch them and move on.

I think ditching a book is easier now because I seldom buy paper
books anymore - deleting an e-book feels like less of a loss, and if
I'm reading something totally new (to me), I'll usually torrent it
or check it out from the library, rather than buy it. If I like the
book, and the author has more books or this was the first of a
series, I'll then purchase any other e-books I read from that
author. First choice is to buy direct from the author's website, if
that is possible, and next is the Kobo store.