It's important to realise that when I'm talking about transaction costs I'm not referring to the inbuilt transaction fees in the protocol - those are bitcoin-denominated and so are an internal transfer within the ecosystem (likewise for the block rewards).
I'm taking about examples like when someone uses bitcoin to transfer money, by converting an external currency into bitcoin, sending the bitcoin somewhere then converting it back into an external currency. Whatever loss of value they see from that overall transaction has been transferred into the bitcoin system as a whole.