Never, you don't lose any progress. You create the world, and it's entirely hosted by players. Either dedicated or pier to pier. Even in future updates you would still be able to play on the first world you made. I don't think they'll ever make it incompatible. You might lose out on new content but mods also allow for regenerating your world.

Nothing would change in your world that you created, buildings, work benches, items, food, etc. Your personal skills & progress levels will remain the same, too. Only things changing would be more added items/enemies/buildables would be added in future.

get it ...GET IT!

progress normally dont resets
only thing that might happen are:

steam cloud messes your saves (so turn that off, or backup your saves)

an update will add new biomes wich are somewhat applied to your old world but only if you did not explore that area before

It isn't so bad that you couldn't start over anyway. I have around 20 worlds and characters.

Starting over lets you appreciate the changes they make to progression. To each their own though.

The devs have made no promises about new content needing a new save. It is always possible. They try to make it so everything will carry on but no one should "expect" that from them.

I'm actually in a weird spot now where I want to start a new world but don't want to get started only to have an update drop and end up starting again after just getting started ha ha. Not sure exactly when that mistlands update might drop.