> As the amount of coins grow the difficulty of calculating hashes increases.
This is wrong. As the amount of coins grow, the amount of coins generated per block decreases (it halves every
~4 years). The difficulty is completely unrelated to the amount of coins in existence and is instead derived from the hashrate: as hashrate increases, difficulty increases, to target a block rate of 1 per 10 minutes.
> Doesn’t this completely undercut the idea of bitcoin as currency?
I don't think so. Right now people can buy bitcoin which is "guaranteed to become more valuable", and yet they still choose to buy things other than bitcoin with their USD.
Why would it be different if their base currency was BTC instead of USD?
Even if you don't think bitcoin is "guaranteed to become more valuable", there are plenty of things that are close approximations to that (pick any good investment), and yet people still choose to buy things other than investments. How do you explain that? Whatever reasoning you use to explain that can be applied verbatim to explain why people will buy things with BTC instead of just holding it.