Physical copies

You're fully prepared for TPP, aren't you?

Yes. I want to own my shit.

In holland they are creating diamonds for data storage

Disc rot is myth. Companies have been trying to stir up this kind of bullshit to make people think going completely digital is a better alternative. I don't think I have to get into why buying digital is a horrible choice for a consumer for anyone with a brain.

I'm starting to see a lot of "Anti-physical media" rhetoric here in there all over the internet. As long as you take care of your discs, they will last for a very long time. I have a very substantial original playstation/sega saturn/dreamcast/ps2 collection, and all of my games still play great with no problems. Keep in mind, some of these games are over two decades old.

What are the advantages to btrfs over something more standard like ext4?

I've only ever had two discs die to disc rot. Both were micro$oft products.

Buy Steam keys physical is still cheaper than buying on Steam. Guess where key stores get their keys from?

Storage method really does not make much of a different in the long run.