The long dev cycle doesn't concern me. If anything, it's promising how many times they've started and restarted this project. Some of the best games ever went through multiple iterations, directorial changes, etc. Look at how much RE4 changed over the course of its troubled development. Look at Metroid Prime.
Now, I'm not saying the history here is similarly promising... but it's not cause for concern, either. It seems clear to me that FFXV was able to jettison much of its baggage thanks to multiple restarts. In the end, it seems they've come up with something lean, clean and focused. Elements like the road trip, the all-male dynamic, the simple goal to reclaim your throne, etc, all suggest to me a purity of concept that was honed by working and reworking the project.
In terms of the overall impact of the plot... If Noctis grows on a personal level over the course of his journey, if his friends grow alongside him, if the pacing is on point, and if everything is resolved in a fitting way, I think it'll deliver. The setup is already strong. I think they might pull it off.