That would be exactly what I mean by "wither". Lack of developers means a lack of updates.
OpenOffice updates are orders of magnitude less impactful than LibreOffice. Pretty much all the developers went with LibreOffice. OpenOffice still works, it's still available, but it is much less vibrant. Much less, alive? Like it's "withered".
You could absolutely fork Lemmy, and of Lemmy improves it's sorting, adds other features, tweaks, improvements, etc. your fork wouldn't include those. If there was enough developer interest, your fork could parallel those changes, or it could even go in a different direction. Without developers though, it would just be stagnant.