Yes, you need Steam for patches. But just open steam one time and let it do.
Ironman and Archievements aren't important, and they work with the steam services.
And the multiplayer,,, The reason behind this is just, because Paradox had problems with the own multiplayer and the steam one works much better.
But okay... It's unimportant. I think we move to much away from the main topic. Paradox didn't develops PoE just published it and don't force Obsidian to anything.
This (bolded) is the key part - to install the game you need an internet connection and the steam client.
If Steam goes bye-bye, you can't reinstall your games. If GOG goes bye-bye, I can use my backed-up installers to continue installing the game as many times as needed.
GOG is an online shop wherein you buy a product and it provides additional after-sales service. Steam is an online shop wherein you buy a service with product as an inseparable pair. If the service goes, your product (the installation file) is useless (AFAIK and would be happy to be corrected).
This is what makes Steam DRM - you need the steam client to install the game. It therefore controls ('manages') the rights of people to install the software (aka DRM).
Just save CK2 on a CD. Without the installer, but you can save the whole game.