Or have very young children.

Very young children are perfectly happy properly dressed and cold.

Parents in nordic countries leave their babies outside bundled up in strollers in the winter to nap.

Babies can't regulate their body temperatures well so no, they are not happy when cold. They are happy in the cold when properly dressed and warm, which I assume you meant.

The key point here with this napping thing that did the rounds is that this is just for short naps. People in Norway/Finland do not leave their babies in a cold environment for long periods of time. Presumably the main reason they do this cold outside nap is that it's easier to get a baby to fall asleep in a pram (safely wrapped up) but very disruptive to then bring it inside, where it will be warm, have to undress it and then inevitably wake it. But I don't know, there are probably also other reasons. But parents in nordic countries are the same as everyone else when it comes to heating their homes appropriately, especially for night time sleeping.

Personally I would imagine even things like eating would be harder if the room is too cold and the infant must always be maximally layered up. It doesn't need to be any harder.

You're right, I don't have or care about pianos.

As for guests, you can turn up the heat when guests come to stay and still see dramatic energy savings, vs running the heat full blast the whole winter.

You should be able to keep the piano’s humidity acceptable at lower temperatures with a whole house humidifier. Installation will pay for itself in a season if it lets you lower your thermostat several degrees. Some brands are more sensitive than others, of course.