It's like betting on Black for a roulette wheel versus betting on a specific number. You're still going to lose money but you lose the money slower by betting on black than a specific number. You need to show that betting on black loses money faster than a specific number to demonstrate that the Jones Act isn't furthering it's goal.
No ships were built because of the jones act, that's the problem. The jones act required people to do B if they wanted to do A, so they stopped doing A. A is intercoastal shipping. Nothing in the jones act encouraged B besides the opportunity to do A.
In your roulette example, the example isn't between a specific number and black. It's the jones act is playing roulette versus the non-jones act where you don't. Are you guaranteed to not have lost money by some other means if you never played roulette? No. But roulette is a game that provably loses over the long run, and so we should stop playing it.
The everything-bagel approach. One of those requirements incentivises US shipbuilding, the other two incentivise other things. Seems like the net effect was less US shipbuilding and a smaller US-flagged fleet. Given those effects, it doesn't seem likely to have increased the number of US merchant seamen either.