Well that's the point isn't it? You don't buy an index fund in order to transact in it.

On-ramp has costs, then transaction has costs. And in my country at least, selling crypto (which is what you're doing when you buy something with it) is a taxable transaction. So now I have to keep all my transactions and report them to HMRC each year if I do that.

It's not going to happen. If people wanted to transact in crypto, they'd be doing it by now, it has had more than enough time.