A slime draws near
If Splatoon was any indication, Splatoon 2 will have very long legs (tentacles?) while Dragon Quest is very frontloaded. While at launch DQ XI will certainly take the win, Splatoon 2 will grind that advantage away over time, even moreso with the limited supply of the switch.
The question is if it can grind enough to surpass DQ XI total sales - and how well the switch version will sell later on since those sales will be added to total sales, after all.
I think it's possible for splatoon 2 to overtake Dragon Quest XI on the long run, but certainly not during launch