The problem with loops is that, with only a handful of exceptions, they necessitate operating on mutable logic. Many languages get by without them, and I think while operating in an expression context it can be good to not have access to them. Instead, functions such as .map(), .filter(), and .reduce() can be used (though I'm not actually a fan of .reduce()). Though, if we think loops are valuable as expressions, I'm open to discussing it.
And yes, do ... in is indeed a hazard. I'm currently running with the following syntax instead:
const result = (
with x = getX()
with y = getY()
with z = x + y
do processZ(z)
)