In general, I agree that allowing American tourists would not affect the economy of Cuba much, if at all. But I have to ask a couple questions about what "could" happen...
#1) If it is allowed to happen, what about cruise lines? I would imagine that there would be an immediate influx of cruises that would make Cuba a stop in the Carribean. Do any stop there right now?
#2) With the proximity to the Keys, would it take some of those vacationers away? Do we have a true idea of the number of US citizens who would choose to go to Cuba to spend a vacation? I know it's a place I'd love to go see some day.
If we had an idea of the sheer numbers that would travel there, we would have a better idea of how much or little ending the travel ban would do for their economy. Nothing will change for the people, until there is a regime change, IMO.