Some of what we see in XVI makes you wonder why we can't get it in XIV. For example, solo hunts that... aren't a complete striking dummy. Combat without telegraphs in normal content. Perhaps even the combat itself. Universal voice acting (granted, they had 7 years to work on this whereas there's only 4 months for XIV). The graphics, which I know they want to scale down to PS4 for FFXIV, but still.
But a lot of things in XVI design are in the same style as XIV actually. The sidequests are pretty similar in design (some may disagree, but as someone that does the sidequests, they tell you about the life of the people in the area and how they are affected by MSQ events in XIV as well). The zones are, well, zones that have loading screens like in XIV. There are scenarios that the main story takes you into that are similar to dungeons in XIV and then there are boss battles that you can do a bit later, similar to trials in XIV. You teleport back and forth between areas and return to a home area regularly like in XIV instead of being forced to travel back and forth on foot.
Did XVI benefit XIV at all already? Yes. We see people biting apples in XVI and that technology was obviously brought over to XIV where we see the scions eating burgers and things. Also possibly the UI changes that XIV previously had such as quest display settings. Probably other things but I haven't thought about them yet.
