> States can't issue their own currency
Of course they can. You're pointing out that federal law currently prohibits it? I already understood that to be the case when I made my point.
> they're all going wherever the nation goes on this one.
Unless they all collectively decide on a different course of action. The constitution was left amendable. It has been done several times already. A simple option is to repeal the 17th and to return to a state assembly controlled federal senate.
How would that benefit anyone?...
The core feature of the senate that makes it so indefensible is how it assigns power to states, not people. That relationship cannot be amended, per article 5 of the us constitution.