In my opinion, alternative A sounds more reasonable. Look at the difference in meaning:

This verb tense describes an ongoing action in the future. So one would probably understand that creating organ replacements with 3D printers will be a common practice in the future. This is a likely scenario, so the sentence makes sense.

This verb tense describes an action that will be completed between now and some point in the future (fifty years from now). So the implied meaning is that creating organ replacements with 3D printers will be completed when we reach this date. It's an odd interpretation because we wouldn't expect a deadline for this action to happen.