Yes, this seems reasonable to me.
To reiterate other people's points and respond to Jon's comments upthread - I agree about the potential for abuse and agree that former Java programmers who haven't learned enough Swift may reach for throws! when they shouldn't - just like they reach for IUOs when they shouldn't because they don't understand nullability. I am not personally afraid of that after seeing many people express concerns about similar language features - such abuses would surely happen in the small, but would not be considered "good swift code" and infect the ecosystem through successful libraries.
As to "why would we add this other than interoperability", others have pointed out that we already have the throws! behavior for core operators like + and a[i] on arrays, we just don't allow anyone to catch and handle the error. The semantic of throws! is perfectly safe: if you don't locally try the error, it locally traps, just like many core operations in Swift implicitly do.
I think that adding throws! and adopting them for these operators would make existing patterns more safe, and improve recoverability for those sorts of errors.
-Chris