https://github.com/rclone/rclone
https://github.com/restic/restic
https://github.com/bcpierce00/unison
The thing with old, critical software is that after some time people don't really want to dig through decades of C code and prefer to write something new using modern tools. Those projects get plenty of support because people actually do want to work on them. If no one wants to work on rsync than what the maintainer is doing now is just prolong it's agony a couple of years. I would say he should do the minimum work, announce end of life date and move on. People that need tools like rsync will develop something.
Also, having critical software depend on one guy is not safe. We should avoid that. If critical software depends on one guy it should be phased out.