They took an open source app I loved, closed the source, added features I don't need (and ads I definitely don't), making it run slower on my 6-year-old Nexus 7 I use as a reader.
I agree with supporting development, but it was feature-complete as an open source app. The roadmap for 3.0, 3.1, and 3.x add nothing of interest to me[1]. I'll happily move to the Book Reader app that someone else mentioned, and contribute anything to it that I can.
[1] https://fbreader.org/content/fbreader-3x-android-planned-fea...
I agree, but it highlights the many-small-cuts problem
$5 for an eReader, then a mapping app, file manager, photo editor, mail client... the list of developers deserving support is longer than most people can afford. Eventually a choice has to be made to prioritise. For many people that will mean that a less-used category of app can be fulfilled from F-Droid instead of paying for a more-polished one.
Is $100 too much for lifetime access to these apps?
Working on open source is fantastic but most realistic projects are sponsored in some way or form. I doubt that sponsorship works works for an eReader app, so this is a good solution.
TLDR; our fellow developer deserves to get paid for their clearly awesome work.