You can't guarantee allies' shipbuilding capacity will remain intact in war, or that they won't redirect their shipping or ship-building capacity for other purposes, and without shipping the US cannot meaningfully act globally. Things are relatively stable now and most major US allies seem to be in pretty secure positions (a tad less so right now, sure, but in general, for some decades) but I don't think the smart money's on us getting through this century without at least one big war, possibly not even caused by or involving any of the powers that seem most-likely to kick something off now, so there's no guarantee the major action would be confined to or even primarily in, say, the Pacific theater, meaning that it'd be a good idea to be prepared for any of our allies to be taken off the table early in a conflict (or even for their being on the other side of it).

I agree that the Jones Act sucks, but I do think there's some valid reason behind trying to do what it tried to do.