I keep the previous download of updates until the next update after succeeds.
So I have a Calibre624 in my downloads. That will be removed when 6.6 is stable. (I also remove Problem downloads as soon as the patch appears)
So looking back, I have the previous version of those tools to fall back to.
Note: Some of the updates are required just because Calibre is now using a newer version of its building blocks (eg Python, QT...) Kovid does post long before this happens so that PI authors are ready