To some extent that was kind of my point. Many well meaning people have tried to make better decisions at various points, but it just increases the amount of unexpected behaviour.
I suspect another better decision wouldn't really help ;)
For such niche behaviour I think I probably fall on the side of bsdmainutils' "cal -b" output. Be byte for byte compatible with old behaviour, and expect people to use something else when they need more accuracy.
Some of the behaviour you advocate for can be implemented using gcal¹, it allows you to use various dates and also specify a manual reformation date with the `--gregorian-reform=<date>` option. It doesn't offer the fuzzy sliding option though, but the manual behaviour is probably good for the people who care about this minutia.