Switch struggles to handle Breath of the Wild already. What makes you think it can handle Luminous?
*rolls eyes* The PS4/Xbox One struggled with Just Cause 3 too. Doesn't mean that there aren't significantly better looking games with just as much going to that run near a locked 30fps.
Anyway, FFXV can be ported to Switch with downgrades. If you played FFXV, you'd know that there isn't TOO much going beyond the the draw distance, character models, and lighting. BotW physics are more impressive than any dynamic non scripted event I've seen in FFXV.
The issue is whether that's the best approach to FFXV being on the platform. IMO, it's not and their better served doing a spinoff in UE4 with similar graphical fidelity and work from there. Therefore they don't have to exactly mirror graphical assets meant for much more powerful consoles, but achieve a very similar look. If they were using a more scalable engine I would say it would be worth the port but not on Luminous, as it struggles on the base PS4 an Xbox One.
They've got plenty of plot holes to fill and a spinoff would be a great place to start. There is very likely to be a Switch revision out in two years to to Nintendo's access to NVIDIA's Tegra chips, so they could wait until then and take advantage of the extra power that introduces if that happens in time