Scratching my head over this as well. Yes, it might diminish casual discovery uptake that the app isn't in the Play Store, but for this target group I think most users would be comfortable downloading the app from Fdroid.
The larger issue with closing down the entire project including notification servers(!?) is probably a tell that there have been other factors weighing on the developer?
Either way, if the source code is openly available maybe others will pick up development in a way that isn't as vulnerable to corporate policy changes.
Oh, never actually tried Bookwyrm, but I'd expected it would have a social aspect as well? That seems like a lost opportunity.
[Edited to add:] Have you had a look at NeoDb? Also a tracker, but apparently with more social aspects —
I only had superficial experience with NeoDb, so can't say with certainty if a Lemmy community and threads for individual books may be better for you.