Primarily, the problem with that is I don't know anywhere that I would choose to spend money that also takes ETH. I also don't really think that what you're describing is good or common.

Common: The middleman is still there in the most common case since most wallets are custodial. There are a ton of recent examples of exchanges freezing accounts, including the very article that we're commenting under, so I don't really get what you're talking about.

Good: It doesn't make sense to use a wildly volatile deflationary token as currency. Deflation discourages spending which is why there is no real currency that restricts the supply of money to a fixed amount, because it encourages hoarding.