In my opinion they should print all the annotations with their respective times to the description of the videos that still have them, with a remark like
"Google Backup of annotations:
03:01-3:35 annotation text content"
"Google Backup of annotations:
03:01-3:35 annotation text content"
I wonder if implementing something like that would be technically feasible.
The third common use that I've seen is linking to other videos, and that's the one that YouTube's replacements systems tackle.
Another somewhat noteworthy thing is that the annotation system is tied to YouTube's channel logo overlay. Once the annotations go, it's likely that the logo will not be hideable anymore.