They don't need to test two architectures, if there is only 32-bit. So that's not a real point. And CPU extensions for a basically shop? Alright the Steam app is not just a store, that's true, but it's not like a game or low level library itself. So I am not sure if additional 64-bit only CPU extensions is a great reason.
I'm not convinced at the moment that this is a huge win. The biggest reason is being dependent on the older libraries, which in some environments is just nasty. I am also not against 64-bit only. It is just surprising how much demanding some users are and how hugely celebrated this is, if it actually doesn't matter much in real world. As said, besides the library thing.